I’m so confused

So umm my mom and step dad have been married for 6 years now and at times they have these really bad arguments which I know they’re gonna have disagreements and stuff but no this is him full on shouting at my mom and always ends up with him running off downstairs for a couple hours, but then when he comes back up he always showers her with kisses and crap and says I’m sorry it’ll never happen again even tho it happens a couple times a week. And then a few months ago he ran off downstairs but left his phone upstairs. He got a notification from kik and then a couple more kept popping in. So me being me I opened it and saw that he has been texting 4 other girls from around the area. And the worst freaking part is on my moms side we struggle with money. My mom can’t work cause her left knee doesn’t bend like at all so she can’t really do that much but anyway back to the story on one of the girls he has been paying hundreds of dollars for this girl to make him personal videos of her and her roommate... a couple nights ago it was just my mom home and when I got in from practice I found her in the laundry room with a bottle of whisky on the floor turns out my brother had left. He had always blamed her for the divorce of our parents and I guess he just couldn’t take it I don’t know. But still my mom had 6 miscarriages before she was able to have my brother so this hurt her more than I could handle so of course I was there for her but then my step dad walked in and acted all sweet and innocent. The sad thing is he got her to calm down more than I did...😔 I just I don’t know what to do she really loves this man and he’s just straight playing her. I’m only 15 here and I wanna tell her I really do but it will break her heart so much and I just I don’t know if I can do that or not.... someone anyone please