Let me give you some backstory. I (29f) moved away from my home country and settled down with my husband (29) in his home country 8 years ago. Completely unrelated to our situation, my dad (50), my stepmom (50) and my younger sisters (much much much younger than me, 13 and 5) moved here as well around 5 years ago.

First I was really happy about this. It allowed me to build a better relationship with my father and to be in my sister's lives, both of whom I truly adore. It was also nice to have some family close by. I also have a good relationship with my stepmom, I used to live with them for a few years in my teenage years so we have some history together. However, since they moved here, she has made no effort to make any new friends or social contacts in her new environment, leaving me to be her only physically available friend. She calls me to complain about her life, my dad's behaviour, work, whatever there is. It is very one-sided and she usually has little to no interest in how I am doing.

My dad really likes my husband and he likes spending time with him, so everything is given for us to spend a lot of time together. We visit each other a lot, do day-trips, go on weekend trips and there is a general pressure to also go on complete summer holidays together as well (we have done it once). Between these and the sometimes daily hour-long calls with my stepmom, this is just way too much for me. All of these activities together are meant for me to emotionally support my stepmom, and I can barely handle my emotions, let alone constantly being there for someone else. 

They have made it pretty clear that next year they want to go on a holiday together again. And I don't want to. First of all, it's not relaxing for me because it's alwaaaays about them and we can't afford two holidays so I would be giving up time to spend together with my husband. 

So this finally pushed me to start setting some hard boundaries. I am really ready to do this; to tell everyone that it's too much and that they need to deal with their own issues. But I am afraid that this will cause a huge fight (it definitely will) and that my stepmom will be angry at me (she definitely will be) and that this may go as far as for them to break all contact with us (my stepmom did that to my brother already). So this fear is paralyzing me. How does someone get over this? 

P.S.: whilst my husband is trying to support me, he would rather go on a holiday with them to avoid an argument. He also feels like that they have done a lot for us in the past few years so we should "repay" them by spending time with them whenever they want it. For me, whatever they have done for us in the past few years does not make up for the pain I am carrying from my childhood, so I do not feel like I owe them anything. Obviously this causing conflict between me and my husband is also not helping.