It wasn't real smart of your husband's kids to accept this woman as family without getting some kind of proof, but it is their prerogative to have a relationship with the woman whether or not your husband wants to and whether or not she ever contacted your husband.  If in fact she is your husband's child, she's a sibling to his children and there's no reason they shouldn't have a relationship.  It's your husband's choice whether to accept the woman as his daughter, but it is his children's decision whether to accept her as their sister.  They're all adults.  Do you know for a fact that she didn't give his son or the other siblings some kind of proof?  I have friends who have connected with half-siblings after getting an Ancestry or 23andme match.  I do think it's odd that his kids didn't reach out to their father about this, but I don't know the relationship history.

Well, I think it's kind of crappy for your stepson to invite her to the Family Facebook group as a unilateral decision, and without thinking about how his father feels, or how his siblings feel. Yes, he can have a relationship with her, but "welcoming her to the family" in that manner, without everyone else's agreement, seems rude.  He could have talked with her and then spoken to his father and the rest of the family before he decides she's legit and invited into the family.

BUT, that being said, this is between your husband, his son, and this new potential daughter.  I'd just stay out of it and advise DH as he asks for it.

I have to say that I can understand how you FEEL about this.  I may be wrong, but it seems as though you have never been accepted into this "family" and yet here comes a complete stranger who is welcomed with open arms.  

I can understand your distress at that.  However, you should also be concerned because your DH better get some hard evidence he is her father.  After all the newness wears off, there may be issues cropping up like inheritance issues etc.

I had a coworker who had a long-lost sibling show up (with actual proof) and her father quickly developed a close relationship with this person (as though to make up for lost time) and it wound up fracturing the entire family.  I'd recommend everyone proceed with caution.  

No DNA, no recognition.  If she is proven through a DNA test to be his daughter, then welcome to the family as an adult.  She should get no support or special treatment or special recognition even is she is genetically verified to be his. Accepted yes, worshiped, no.

If she isn't his... then she and everyone else who have engaged with her should be aggressively confronted for their naive and manipulative bullshit.

If I were your DH and this woman was proven to NOT be my child I would go after her and her mother with every legal tool at my disposal.  It would be incredibly financially painful for them.

If she were proven to be my child, I would go after her mother with every legal tool at my disposal for denying me a relationship with my child.  I would destroy this woman's mother financially, socially, and extremely publicly

I would bring the pain to the fullest possible extent whichever way this turned out once verified genetically.  

What will be telling is if this mystery 40yo will submit to a DNA test.  If she won't, she gets zero recognition IMHO and anyone who has accepted her needs their noses rubbed in their naive bullshit.

If this woman had any character rather than contacting your DH's grandson, she would have contacted your DH directly. Even if she were proven to be his, If I were your DH I would jerk a knot in her tail for that lack of character and courage and let her know that though she is his genetically if she pulls that kind of crap again she will never be accepted.

She made no attempt to contact him.  Instead she pulled sneaky bullshit.  I am not a fan either way this DNA test goes.

Just my thoughts of course.

This sounds fishy.

Until you have proof, DO NOT let this shake you. It might be nothing more than a mirage or a lie, as those steeped in reality call it.

In your shoes, I'd talk to my husband and try to make an agreement of how this will be handled by the two of you now and going forward -- regardless of what anyone else does.

This woman has not reached out to your husband, so he's under no obligation to do anything now. If she does, that would be a good time to get your DNA proof and then decide WITH YOUR HUSBAND how this will be handled.

In the meantime, forget about it until it comes home to roost. If she's a scammer, she'll go away. If she's not, she'll find her way to your husband and then you can worry about it. But this might just be some games to shake you up, so don't be their fool. Keep your head, be logical and make decisions between you and your husband.

If your husband wants to jump in head first with this woman, remind him you know nothing about her. Tell him you're willing to address it when you know she belongs in your family. Until then, she could be a scammer or any other crazy out there, so you must insist he keep her at arms' length.

The step son was out of line but you can't control what he does or who he invites in his life or FB. Have a talk with H and see what he wants to do moving forward. Does he want to take a paternity, if it is his daughter what does he want to do next? Does he want to have a relationship with her or not? If being on the family FB group really bothers you, remove yourself.