Family issues

I'm only 15 but my mother has been telling me about how she thinks my dad doesn't like her anymore. She's been the only one talking to us about it which I also don't think she should be doing. I want to stay in my bubble and think we have the perfect family. My dad works hard at a job he doesn't like but I pays well, we live in a big house because of it. He's the one with the money, while my mom works too she also spends a lot more than he does. The other day my sister had some friends over while my parents and brother were at the beach. They brought alcohol and were drinking at 18. She expects me to be ok with it and I know I can't tell my parents or she will get in a lot of trouble. If I tell on her she most likely won't talk to me for a long time. I also found out that night when my sister had gone through my brothers iPad on my moms request she found that he had watched porn, but she didn't tell mom. She said she didn't tell because he was already upset about having his ipad looked through. My brother is only 10. Also she's thrown me under the bus for that a few years back too. I wasn't that young but she did tell on me. She tells me that I can't tell my parents about any of this and I don't know what to do because if I do my relationship with my sister might be ruined. This was the first time I'd ever seen her drink and apparently it wasn't the first time. And I'm tired of my mom telling me about how thinks change after 20 years of marriage.