Feeling overwhelmed

I have a step daughter that is absolutely the greatest. I love her and she loves me. The thing im feeling upset about is her homework. No one helps her with it. Custody is 50/50 so mom has her 2 days and we have her 2 days a week and then everyother weekend is alternated. Last year step dad and mom did the same thing. Mom never helped with homework on her days it was step dad and he would half ass it. Step daughter would come over with it done wrong like no one helped her but homework was signed off like it had been done and checked. They never read so i had to do twice the reading redo the stuff done on their time and do double the work for her learning her sight words just so homework could be completed. My SO was great last year with the homework but this year he just doesnt get it. I understand as the teacher is very unorganized and not very put together as we saw at back to school night. My SO just isnt good with the homework aspect as he never has been even when he was in school himself from a very young age. So i take over that part for him because he knows i will make sure that everything is done correctly. I did alot of overtime last year to help step daughter catch up because her mom and step dad werent doing their part on their time. Now its the same thing all over again. Its overwhelming and sad that they dont put the effort in to help her. I guess i would feel differently if we just had step daughter during the week that way i could make sure it was all done each day instead of playing catch up the 2 days we do have her. My SO does what he can to take some of the responsibility off but as a mother to this child i get anxious and need to make sure it is all done so that she doesnt fall behind again. Talking to the mom does nothing except start problems so thats out of the question. I dont know what to do anymore and i will not let her go without doing homework because of mom and step dads inability to care for her education. Im stressing.