There was co-worker1 who was flamboyantly gay. And randomly in a conversation she outed co-worker2 to me. And at that time I remember being really mad on behalf of co-worker2. I didn’t make a big deal out of it, just made hmm hmm noises like it was no biggie then put it in a corner of my mind. Co-worker2 came out to me years later and I finally told him I already knew (actually forgot about him being gay because how much time do you really think about co-worker’s sex life?) But it boiled down to the same issue. Unless they’re openly out of the closet, it’s really bad form to share that news. Being gay himself isn’t a good enough excuse. Being related by blood still isn’t a good enough excuse. 

Tough situation but I think the best course is for DH to pretend that news doesn’t exist until your daughter tells him. He can tell her he already knew and doesn’t care later. But give her the time to mentally be ready to share rather than the traumatic shove out the closet.  

I would ask DH not to say anything and I wouldn't say anything to DD. As it will only hurt her and make her feel bad. Since this is a sensitive subject and she is probably already feeling insecure I wouldn't add to it by letting her know about SDs betrayal.

I wouldn't want to tell DD she'd already been outed, but I would try to find a way to let her know that her sister is not trustworthy. 

It wasn't your SD's place to say anything, and I would suggest that your DH doesn't, until your daughter broaches the subject, or if he does, do it in a very low key way.  At 13, a person's sexuality is not settled yet, and young teens often have gay fantasies as well as straight ones. My SD24 said she was bisexual at about the same age as your daughter is - however, she has never had a girlfriend (not that anyone would have minded a bit) and has had several long term boyfriends. 

I think it was wrong of SD to tell her sister's private information, for sure. But I'm wondering - why is DD afraid to tell Dad? I don't have kids but I'd have a hard time not telling my husband such an important thing about our child.  Will he be upset with her? 

Now that he knows, I would definitely have him tell her he knows (if he's going to be supportive and not upset about it) - that will take some worry off of her mind about him knowing.

Also, what Kes said - she's very young, and it's a thing nowadays for kids to think they are bisexual at a very early age. It's good to be open about it, but there's a good chance it will change as she gets older.

I think daughter has to be told SD outed her. Otherwise it could be seen as everyone kept the secret from her and adds more flame to the fire. 

Personally I think you need to talk to your DH first address the issue and then let him know that SD outed her sister and had no right to do that.
Next talk to your BD and let her know going forward she needs to be more careful when talking about her future sex life with family members as they don't always have her best interests at heart.
There should be no secrets when it comes to deception, and SD needs to be held accountable for her actions.

OP, your DH needs to put his foot up your SD's backside for telling him, and then you both need to let DD know that her trust was betrayed.

While some kids come out and hope for the shock value, most are legitimately worried about how their family with react. My OSS was worried, which is why it took him a few years to come out as gay. We've all been very supportive, and it has been a HUGE relief to him. My family, though? My mom reacted poorly (at the time) to me coming out as bisexual. My sister is a lesbian, and it has been rough for her since our family acts supportive but says/does very unsupportive things. 

Ultimately, it's a crapshoot, and many in the LGBT community assume the worst. Even if they don't have a reason to believe that their family and friends would react poorly, the fact that folks HAVE been killed and HAVE been disowned and HAVE been put into conversion therapy is enough to make them be anything from hostile to sheepish. 

So, OP, this needs to be addressed head-on and not secretly. Talk to your DD and make is abundantly clear to your DH that he needs to address this harshly with SD. DD will have to decide how she wants to handle SD, but SD did something that can be potentially dangerous. And SD sounds old enough to know better, or at least old enough to be told in no uncertain terms that she effed up.

Also, I'm very disappointed in these comments. I'm bisexual. Didn't come out until I was a young adult in spite of knowing for several years that I was. My relationships with women have been casual, though most of the relationships I've had with men have been, too. If you're a bisexual woman, the pool of eligible men is quite larger than the pool of eligible women, so it shouldn't be that surprising to folks that bi women tend to have an easier time dating the opposite sex more readily/easy. Add in that bisexual folks are not readily accepted in the LGBT community (you aren't gay enough) and end up fetishized by the straight community (particulary for women - threesomes), and it's totally reasonable that bisexual folks would "pick a side" romantically. The people in this thread discounting others' sexuality based on behavior is cruel. It hurts - really effing badly - to be told that you aren't "really" bisexual because your behaviors don't match what people think it should.

OMG what a crap thing for your stepdaughter to do.

I would be MORE than angry...whats more than angry??, I don't know. What I do know is,  It would be a cold day in HeLL before she was welcome (your stepdaughter) into my home. Step kid is a threat to the mental health of your daughter. Whats she gonna do next? Video her and send it out on social media?

Teens who believe they are trans, bi, gay are very vulnerable.

((HUGS)) and deep compassion for you and your daughter.

May you find the wisdom to maneuver thru then next days, weeks and months. May you have the clear vision to protect your daughter. NOT to be sucked into poor dh and his poor kid who may be viewed as making a BAD CHOICE....barf.

Your DD needs to be told the SD outed her to her father.  He needs to discuss the situation with her.  Otherwise, it is everyone covering and protected SD.  I would be pissed if I found out later that my family didn't let me know.  It would be my hill to die on.

It is most definitely a big deal! That's a highly personal thing, maybe the most personal thing in the world. No one has a right to out  someone else! 
The only thing I can think, to preserve your daughter.'s feelings, is to ask your DH to pretend he doesn't know, until your daughter chooses to tell him. A lot of people have horror stories about coming out, and you don't want your hateful, gossipy SD to be your daughter's horror story.  No matter how much you want your daughter to know what that brat did to her. If this turns into a shit show,  it will give your SD exactly what she wanted. What a hateful little jerk.

Our son (my former SS-28 who asked me to adopt him when he was 22) came out to his mom and I when he was 20.  His mom was taken totally by surprise.  I was not.  He and I had discussed it a few years before and he asked me not to tell his mom.  I had walked in on him viewing gay porn.  He claimed it had just popped up after he hit the wrong link.  I told him that his mom and I were there for him if he needed to discuss things with us. He was adamate that he wasn't gay and begged me not to tell his mom he was surfing porn.  I respected his request.

When he came out he threw my butt under the bus.  He called his mom and I, after a lot of "umms" and "ahhs" I told him that we had his back and to just tell us what was up.  He told us. Then when is mom was surprised he said "Mom, you had to know. Dad has known for a few years and he tells you everything."  Nope, I kept the confidence he asked me to keep.  This was not and is not my story to tell, nor is it his mom's.  It is his story.  We have let him evolve who he tells.  I do believe it is his story to tell... However, .......

When he came out I told him that his mom and I love him very much and pointed out that not everyone would be accepting of him as a gay man and asked him to be save and aware of his surroundings.

His mom regrets that she did not know.  She feels that she could have been supportive of him and could have helped him was he went through the self reproach he had for himself and the struggles he went through thinking that there was something wrong with him.  These are things that we have learned over the years since he shared with us.

I am on the fence as far as "outing" goes.  Family should know.  In hind site and if I could take a do-over I would have shared with his mom even though he asked me not to.  If informing a parent is not done with malace, I could see it as something that might should happen.  If malace is at the heart of it... then no.  In the case of malace outing someone is wrong.