What to do about my family...advice needed please?

OK So im 15 years old and i have a 19 year old sister who just can't communicate with my parents...mostly with my dad though. She hasn't had the best relationship with him since she was 5. Mostly because he cheated on my mom and almost left us and he used to yell at her for everything and just ignored her. But That was a while ago and they all ended up going to family counseling for all that, and my parents, even though my dad cheated on my mom, still stayed together. But now some of those problems seem to be coming up again and other problems. I try to help my sister out but it just isn't enough. My sister is always getting into arguments with my dad and then when my mom tries to help she gets stuck in the middle and next thing you know shes crying and I'm trying to comfort her but i don't know how! Now its gotten so bad that my dad is just blaming everything on my mom and my mom is starting to say that if things don't start looking up then either my dads going to leave or she might leave or send my sister to Mexico so my dad and my sister can see how its like with out each other. But i don't want any of them to leave...like i know sooner or later my sister will move out because she just graduated and its getting serious with her b/f but i don't want her to move all the way to Mexico! I just feel stuck between my sister my mom and my dad but who I'm really scared for is my 10 year old baby brother...he adores my dad and everything...since my dad is the only man in the house. But i just don't know what to do...i don't know if a family counselor or psychologist will help. And im scared that if i talk to my dad i will only make things worse. Please can anyone give me some advice for this kind of problem because i feel like im running out of options!What to do about my family...advice needed please?
thats a good idea