Being a Step parent

Words of encouragement or advice will be helpful. Daughter just started kindergarten this year. She has started having homework about a month ago. My SO and i have been doing the homework with her every week. I asked her last week who helps with her homework when she is with her mom and she said no one she only does homework with us. SO and i already kinda thought that but she just confirmed it. She has to read and be read to everyday and go over words to learn how to read and no one is helping her with that. She said they just write stuff down and sign but dont actually do it. Her mother wants full custody but doesnt care about helping her learn. Her homework is done wrong because she does it by herself and we are the ones to help her fix it so it is done correctly. And it is stated on the front of her homework packet that vhilds homwork is to be done with an adult not by themselves because obviously she cant read the directions given gor homework. I just dont understand as a stepmom i want daughter to succeed because she is smart but her mom isnt helping and neither is her husband. I pray that we get 50/50 custody so we can make sure she grows up well educated and respects herself and others. Things she doesnt learn from her mother. We would never go for full custody because we believe in being fair to BM regardless of how she is and daughter needs her in her life because she is her mother. I just hope wecan make a difference in daughters life so she foesnt turn out like her mom because her mom is a horrible person. I wont go into detail because thats a long story. But please if anyone can help with advice on what we can do to better our chances or just encouragement that we are doing the right thing will be helpful.