you don’t need a older SD who crazy influence your kids.  It will only be drama 24/7.  At 12 or 13. You are not going to have any influence in SD life.  She is not going to be part of your family, she doesn’t want it. 

This is super hard, I know. But this isn't a new SD - this is a child who has been around for longer than your relationship. This is a child who has basically been abandoned by both her parents. This is your husband's daughter even if he hasn't acted like a father. So of course she's going to have issues. 

You were super young when you got together so I guess it would be harsh to judge the 17yo you. But you are almost 30 now so you already know that things don't magically happen just because you want them to. She isn't going away and she needs help. As for the money part, yes, your DH has an obligation to this child. 

You have to decide if you want to split with your husband (and you already know what kind of non-custodial dad he is) or if you want to make the best of this. 

I strongly suggest counselling to help you manage your feelings of resentment - some of which are justified and some aren't. If you think counselling is expensive then try divorce. 

1st I would talk to BM's mom. What legal paperwork does she have for custody? Why hasn't the grandma adopted SD at this point? I say this because teenagers are expensive, kids are expensive. It sounds like all of this may be a grab for child support. 

2nd- get prepared. I realize that grandma is young because all of you are. However, on any given day this kid can show up at your door and you get her full time. If something happens to grandma, and BM is a junkie, you win the SD prize. 

3rd- before she comes to stay with you for weekends, summers, etc- demand that she be in therapy or if she already is, DH needs to start going with her and then you need to be added to the mix. You guys need to work through this very complicated matter together before you thrust it on your other children. 

4th- at your house there are boundaries and rules. SD is not special. She doesn't get a pass on her night to do the dishes (or whatever other basic chores your kids have). This includes consequences for actions. I am in control of my kids phones and social media. At any given time all of the kids have to hand me the phone, I have all codes to unlock anything, I take it to my room and go through it. It might happen 2x in one month, it might not happen again for 8 months. But it can happen anytime. If she is posting dangerous things to social media (cutting), that requires a consequence that a therapist can help with and maybe even in-patient treatment if it is serious enough. Which with a horrible mother, and absent father, SM that doesn't want her (trust me I get it, but does she?), and living with grandma-- maybe things rolling around her head are that bad. 

On a side note, my son, while yes, his father and I are divorced, has a great life. Me and the ex communicate and support eachothers households. My hubs has a solid relationship with both my kids, my son gets along with all of his sibilings, 1/2 sibilings and steps. He excells in school, plays basketball and golf, has friends. On the surface everything is great. However, he suffers from major anxiety and panic attacks. He came to me and told me he wanted to kill himself. It wasn't for attention, he was scared. He didn't want to feel this way. We went to counseling, still going to counseling, just started him on meds (which is scary too). My point is, my son doesn't even have all the shit that your SD has going on in life. However, depression and anxiety and those feelings of wanting to just end the sadness are real for all kinds of people and you need to tread lightly.

5th- Sure, you might wait to do some of the bigger ticket family fun things so that she can be there, but it doesn't mean the only time you have fun is when she is there either. 

I don't blame you for being upset about it, your feelings are legitimate too. I give your DH credit for stepping up for this daughter he created when he was way too young, but he really should take it very slowly, and keep not only SD's needs, but your needs and your other kids' needs in mind, too.  She should be in therapy and the rules and expectations of your home should be very clear.  Visits should start slowly, not suddenly every other weekend.

This is why 14-year-old parents should give kids up for adoption, not have the grandmother raise them.  I know that's hard to do, but now this kid is damaged by all this instability. If DH asks her to stay with grandmother, she'll feel abandoned and unwanted, and as if his other kids are more important to him. 

It appears that you were going to have to deal with this when the girl turns 18 anyway, correct?

It doesn't really matter. Your situation sucks. So does her situation. So does your husband's situation -- someone should have insisted on an abortion or adoption 12 years ago because the decision to have a kid no one wants has now come back to haunt everyone.

While this isn't your fault, it is your problem. All you can do is leave or put your best foot forward in the situation. You can resent it, but I still think you have to force a degree of compassion for her -- the thing you share with her is that you are the only two who bear no blame in this mess.

If you put forth your best, maybe this can be a success story -- maybe the kid needs love, discipline and stability. I'm not optimistic, but in your situation, it's the best you can do short of leaving.

This is a really sad story and I'm sorry you're going through it. Try to draw on your sense of humanity and see what you can do with her. Success stories are rare, but not impossible.

A few questions for you, when I read this my heart just sank....I was in the same situation as this young girl the story many years ago it is so erely similar and your thinking/feelings is almost the exact same as my SM's was many years ago. I was rejected from the resentment my SM held onto and could not let go of resulting in me being sent to be raised by other family because my father did choose to stand by her decision and ultimately lead to their divorce years later from what I understand...I still have never spoken to any of them since being sent away all those years ago...I can possibly help you with some insite from the SD perspective to help you have a better outcome for your family, granted many years difference along with different generation beliefs....but I promise it isn't too late to help her if you choose to... ultimately this decision can pave your future effecting those closest around you.

How is the SD with you? Set aside the resentment feeling you have when answering or at least try.

Can you ever see yourself wanting to form a relationship with her?

What are her positive qualities?

Why does she choose to act out? 

When acting out is that her only attention she gets if so, is that why she does it?

What makes her happy?

What does she think of her siblings?

What does she think of her father coming into the picture after all these years?

Is she willing to show you what she is capable of if given the chance?

How/what does the SD feel/think knowing her father has another family?

What is she wanting to see in the future for her and her possible relationship with her father, siblings and you?

Why does she cut?

What is important to her?

How does she see her self-worth and why?

What helps her knowing/understanding she is accepted?

For me growing up it was hard..like any "normal" child I just wanted a mother or father to accept me or want me. For a very brief time I did live with my mother who was an addict/alcoholic. I thought as long as I was the "little mother" in the house helping raise my little brother I could stay. Cleaning the house,cooking, making sure brother was bathed and had clean clothes, trying to make good grades, never asking for friends to come over,staying out of the way by any way possible would allow me to stay. It didn't. I was suicidal more times than not depressed all the time....

So I understand the hardship this young girl is going through more so....I can only pray you choose to open your heart to the possibilities.

I just spent 3 hours plus writting to you going through reading it so that it made sense lol then I guess it timed me out and I lost everything I put in there...I'm sorry. I am fairly new to this and doing it from my phone rather than the computer. When I hit send it said I am not authorized to send messages to this user? So I am going to type it out again may take me a bit.