If your good with relationships in families and dont mind a long read this is for you?

Ok So im 15 ,I got a little brother who is 13 and my dad a.When we were young my mom left us to go to a foreign country for work, she never came back for the first 3 months of her going she used to ring every weekend and told us she loved us and can't wait to come home ,but when the weekend calls stopped and no money to feed us or to pay her role as a mom came in something was up .Dad rang night upon night, sent endless letters to her, and after 6 months of confusion she replied and 'met' some one else she was gone for about 6 months we couldn't visit her we hadn't the money, and dad knew something was up and i remember me and my brother being so excited to get a letter in the door from her because we thought she was coming back and it wouldn't be long before she was back to be with us .This happened about 8 years ago.i was 7 it think at the time i can remember parts of it even tho it was kind of long ago.She hurt dad so much at the time it hurt me too ,because i could never see such things happen in my life as painful as that it was my mom the person who should always be there for us .Dad then got depression for a few years i never knew as i was a child then but now i understand.He never dated ,never trusted women and never went out instead he worked slept and cooked for most the 4 years he has been the most amazing father .Luckily four years ago he met the second love of his life.There was a 10 year age gap between them but it dosen't matter,usually stepmothers were seen as the wicked women of the house but i could tell she wasn't. Dad knew her about a year before they got into a relationship and he asked me and my brother would it be ok with us if he brought a woman into our lives which was fair.They then married about a year and a half ago and i could see dad was changing because she made him so happy it got him out of depression .They had there first baby together last december luckily it was the first week of december because we got to spend our first christmas with our new step sister and shes all our world! :) But now here is were the trouble starts ,last month unexpectedly my mom yes my mom came back.shes ruined everything between us and im in the middle because we are her children she expects us to go around and spend time with her, ever since shes came her jealousy towards my stepmother is overrated she trying to get me and my stepbrother to turn against her which makes me mad but on the other had its my mom who i never say since the age 7 .To make this story short my mother brainwashed my stepmother into things telling her things that dad did WHICH he never done.She then told me that before we know it i'll be back in your lives in no time and told me that she took my stepmother out with her for a night out and told her my dad abused her and cheated and thats why they ended it and that she never told the kids that us! When my mom told me ya i was mad and i told her to leave us alone and i told her everything that was on my mind and i said out of anger she is not my mom.when i went home my dad and stepmother were constantly fighting and she left she cant believe my dad because my mother told her so much and it messed her head so much .shes gone four days to work things out and im afraid she wont come back.I don't want my mother in our lives anymore she hurt us so much. i know i should have told her at the start to leave us alone, but i was so overwhelmed with the idea of her here now shes upset everything and our family was finally becoming a great family but its divided again.i miss my stepmom and my little sister .My dad did not want her anyway near his wife and us but my stepmom thought it was of the past and that everyone should try to reconnect she always saw the good in everyone and now shes burnt!My stepmother won't talk to my dad and he is devastated and REALLY upset how do i get her to listen to me? i want to do this not just for me but for my dad and i dont want my half sister growing up with one parent like my brother and i did! please help if you can i want to make things right again.