Hi todayistheday,
I hope your week vacation with your sister turns out to be a good one.
Reading your post, I recognized anxiety, which I think we all suffer from.
I think we all sometimes ruminate on old baggage, and what could go wrong. This has always been one of my big problems, but is something I'm currently working on. I can see now, how unhelpful it is when I "worry" about things that might or might not happen in the future. Anxiety is so negative and unhealthy-feeling. It sounds like that is what is happening, since your vacation starts tonight.
It sounds like you don't
want to talk about your mom. It sounds like you aren't sure if it would feel supportive, or be triggering. The truth is probably that it could be either or both.
I think if you know in advance that you don't want to talk about it, you can gently say so. On the other hand, if you are confident that talking about it could be mutually supportive, then could you maybe limit how much time you spend on the topic...? Sometimes these conversations start out ok, but then they can either go on forever, or take a detour to the darker side. Maybe that would be the time to change the topic or the activity. Of course triggers can happen at any time and unexpectedly.
I think you have options. Just trust your gut todayistheday. If it feels right (supportive) to talk about it, and you
want to talk about it, then do so, but watch out for when it starts to feel like your emotions are amping up, and switch it up by doing a fun vacation activity. If on the other hand, you already know you
don't want to talk about, then don't. It's a vacation, so maybe you just want to do "happy" things.
My T told me that worrying about the future brings anxiety, and thinking about the past brings depression. So it's better to live in the moment. That helped me a lot to hear it put like that. I don't know if it's as meaningful for others, but I thought I would mention it.
I hope you have a really great vacation.

It sounds like they have been good in the past, so cheering you on to have another good one! Enjoy!