Blended family when married

I've been married to my husband since March, I have an almost 4 year old and he has a almost 2 year old and a 6 month old (he broke up with their mom while she was pregnant cause she was controlling and very emotionally abusive towards him) 
Well I had absolutely no problem getting step kids, I loved them before we got married. But we have to live with his mom till we can afford a bigger house and I swear she favorites his oldest daughter and it drives me insane! She will only offer to take her places with her never all 3 or any of the other 2. She'll buy he tons and tons and tons of clothes but only will buy the other 2 like 1 t shirt. The oldest one is so mean to her little sister like trying to hurt her mean and she don't even do anything!!! If my husband wants to play something my son is doing like if we go to Chuck e cheese his mom always says "really you're just gonna leave Addison here and go play with him?" 
I swear I'm at my wits end!! My husband even knows she favorites her and has tries talking to her and she never listens she always thinks we're always attacking her -___-