Sick of step daughters mother/ husbands ex! she is an absolute control freak and im sick of it! what to do?

Ok heres how it is, I am married have a baby with my husband and he has a daughter from a previous relationship, they broke up when she was 1 and we got together when his daughter was 2, she is now 6 and her mother and i have had many fallings out through the years, (they werent face-to-face they were through my husband and in all honesty they werent started by me!) she is always manipulating her daughter, i pick my step daughter up from school half the week and the mother picks her up the other half, i have become very attached to my stepdaughter,i pick her up from school because it helps my husband and my son gets to play with his sister too, the mother lets me do it cause she has 4 other children so it helps her even though shes not happy about it at all, she plays games, like once before we had ever fallen out my husband asked me to go pick her up from the mother because it was our night to have her (he couldnt), so i went there and my step daughter through a tantrum saying that she didnt want to go with me, in the end after her mother called my husband and he insisted that she come with me cause we had plans the mother put her in my car but wasnt happy about it, then in the car after my step daughter had calmed down she told me she wanted to go with her mummy (at the time she was 4) because she said she was going to take her up to the shops with one of her other brothers and buy her a treat (her mother obviously planned this to prove a point that i shouldnt be picking her up) afterward she rung my husband having a go at him about her going with someone she doesnt like which is completely untrue and the mother knows this too, she does these sorts of things all the time, and the thing is she always needs me to pick her up when she can't so its all when it suits her, but the thing i want to know is what to do now i am at my wits end with all these mind games i know when my step daughter is older her mum is going to manipulate her in2 hateing me, i can see it starting now, we are both doing the same course but im doing most from home so i see my son she said today "mummy said shes going to real university not on the computer" i just said really.. Alot happened today i better not get into it all ill be here all day but im seriously over all of it i love my husband but i dont know if i can do with her screwing with our lives anymore (trust me there is way more to it than this she is really sticking the knife in anytime she gets!!!) is there anyone out there in a similar situation if so how did you deal with it!? thanks for any serious answers