Do not under any circumstances downplay or degrade your relationship with your BD. That would be so unfair.

They are different children. Even kids with the same bio parents can be as different as day and night. I know two sisters who could not be more different - only a couple of years difference in age. The tall blonde one was a dropout, had two babies from different baby daddies who disappeared and was involved with drugs and some bad stuff. The short dark one just got her phd in a very hard science oriented field. But their parents love them both and the two sisters love each other.

The older one who had so many problems is now married and very settled and responsible. The younger one is having a blast being single and carefree. Different paths for both of them.

Don't downplay relationship. Maybe be the catalyst for change for her. How did she flunk? Maybe she's been second string to everything her mom does. In otherwords ask her if she needs help or what she would like to do. Don't be surprised if she initially says no or nothing.

I have a similar thing. My BD12 is beautiful, bright, creative, and well-behaved. I have always made a special effort to do things with my BD and make every minute count.
My SD11 on the other hand was an unwanted pregnancy and now each parent tolerates her for 50% of the time. She is seriously overweight, has no self-esteem, she does good in school, but when she doesn't she lies about it and then cries when she gets found out. She is too lazy for sports, too lazy for a musical instrument. She seems depressed to me. I have butted heads with this kid from day one-for 5 years, but not too long ago this changed.
Luckily, I have my BD all but 8 days a month, and only have SD for half the time, so when I have BD by herself, I do extra special things with her, and when I have both kids, I do things with both of them. I DO GET SICK OF SD because she is around 7 days on/off but if I want to do something with just BD and SD is here I do it.
Thing is, neither of my SD's parents really give a sh*t about spending time with her, nor are they going to help her lose weight or get her the counseling she needs. Some days my DH says 5 words to the kid.
So my advice to you is that since you have your daughter most of the time, you can afford to give up some time to SD13. You will likely find she will be less of a pain in the ass if she gets some attention from you. If there are certain activities that you absolutely do not want to include SD13 in, fine, but you could have a heart and include her some. Start living like a "family" and that will also likely bring you and your DH closer.

I have a similar thing. My StepSon and BS are very different. It is not my responsibility to parent my SS. I do what I do out of kindness. But, this ship sailed on that kid. His parents did him a disservice not pushing him a little bit. I can't undo what they did.

I can encourage him, but I am not mom and I have no standing. Your DH needs to parent his kid and you continue to parent yours.

Same here. SD13, BD13 - skid here 50%, BD here 80%.

I do not feel its my duty to run SD around. She has 2 parents. Her mother is less than 10 mins from us. Its DH's choice not to do anything with his daughter and let SS8 consume him. I do feel SD13 is a bit of a lost soul here sometimes, but i have BD13\BD9 to contend with. They each are allowed 2 activities which require me to take them to and from. I enjoy spending time with my BDs. SD13 is ok, don't get me wrong, but I will not downplay the relationship my BDs and I have.

DH has flat out said to me that he wishes I could have that relationship with SD13 - She has a mother. its not my fault her lack of or the type of relationship she has with her mother. I am kind, I help her out if she needs something that DH really can't handle (getting her feminine products, hair supplies, makeup etc) but consider me a friend at best.

DH was 'upset' that BM wouldn't commit to taking SD13 to cheer 3 days a week. He implied I should do it, I simply said I am happy to be used for emergencies when BM or yourself can't take her, but I will not commit, I have BDs in activities and this cheer doesn't work into my schedule. I know he felt I favor my kids - well I do... and its not wrong. BM the sloth can take her, why should I be committing more time than either DH or BM will for their own child.

My XH doesn't help me with my girls, he is also 10 mins away. I don't ask their SM to drive them around to things I sign them up for - no, I take them myself or I don't sign them up - its not the job of my girls SM to cart them around.

The way I see it, I am a single mom to my girls as far as finances, activities, school events, school, appointments is concerned AND I work full time, my XH doesn't pitch in for any of it - he sees them every other weekend, feeds them while they are there, and that is the extent of his involvement. I do EVERYTHING else.

SS8 and SD13 have 2 functioning LAZY parents - and that is not my fault nor my responsibility.

Agreed... don't do anything to lessen your relationship with your BD.

BUT. Do try to strengthen your relationship with your SD. Even though she is not your biokid, she is still your kid in a way, and she still craves the same things your BD does -- love, closeness, companionship, respect, honesty, and security.

I don't, myself, understand how someone can't love their spouse's child "the same way" as their own child, and so I'm hesitant to comment on that part of your post, OP... I love my SD as though she were mine, regardless of whose genes make up her DNA, because she is a child who needs love most of all. I think, perhaps, you should take some time to think over whether your thoughts and feelings about the comparisons between the two children's (BD and SD) lifestyles may be affecting your judgment regarding the relationship you have with your SD. At the risk of potentially upsetting you, is it possible that because you don't respect her mother or other household or some of the decisions she makes about school, you unconsciously shun her? You don't want to be associated with her because you're afraid it will reflect badly on you, DH, or BD?

If your talk with yourself is honest, understand that those feelings are really, truly, and completely normal. You didn't ask for a readymade family when you married in, but let's face it... you married in And you have the ability to not only improve the environment in your own household when SD is around, but also... you have the ability to improve HER life and HER view of the world. You can affect this child's ability to do for herself. You can affect this child's self-esteem (both positively and negatively). So ask yourself: "your" child or not, would you ever purposefully go out of your way to hurt a child?

By not loving her and treating her as part of your family as she truly is... you're hurting her. I know that's a lot to think about coming from a complete stranger, and you might sit there and tell your computer screen, "Well, what does she know about MY family? What does she know about MY situation? What does she know about MY marriage?" The answer is easy: nothing. But I do know something about kids. They can be fragile when faced with someone whom they want to connect with but are unallowed to do so. The thing about love is that it's endless and depthless; you can choose to give as much as you want away and never worry that you're going to run out.

My advice to you is to accept that child into your family, love her as though she were your own, and strive to help her shore up her deficiencies so that your presence in her life means something to her.

Good luck, OP.