(It sounds really bad because it is)

My sister (20) and I joined our stepmom's family in November when our dad remarried. Stepmom has a daughter and two brothers (19 and 23). I love them all like "real" family, because they are. Anyway, around Christmas our older stepbrother visited for a few weeks. He and my sister hung out almost 24/7, being the only 2 in the house who drinks (they both make it a great part of their personalities btw).

They slowly started acting weird, snuggling on the couch and going on walks together. On Christmas Eve they told us that they had started dating, to the horror of our parents. Stepmom warned them that the relationship would unravel the family if it failed. They assured us that they won't hurt each other and already started talking about marriage and kids. It weirded us all out but we stayed quiet.

As soon as stepbrother went back home they started fighting about the smallest things. Stepbrother has a lot of past trauma with gfs cheating on him and took every missed call personally. By the middle of January they split up, and stepmom was right. Everyone started taking sides and the house became very tense. Start of April everything settled down and things started going back to normal.

Fast forward to a week ago and I'm up early to make myself tea. While waiting for the kettle, I hear the younger Stepbrother's door open and footsteps. He's also not up this early usually, so I was curious. I glanced down the hall to see my sister walking to her room wearing his jacket. I tried to ignore it but I started noticing them becoming a lot more buddy buddy after that, if you know what I mean.

I am very VERY weirded out and I don't know exactly how to handle this. There is no way this possible "relationship" will not tear the family apart at the already fragile seams. Sister and stepmom's relationship is already a walk-on-eggshells situation, and mom finding out sister is making moves on her other son might just send them over the edge.