maybe you can't get off from work.
Maybe you and your sister both need to say, "We never get to spend any time, just us! I'm sorry, Mom, but I'm not interested in taking off work for a big family reunion. I only have so much time, and I'm not willing to spend it on some other sort of vacation."
She doesn't suffer any negative consequences for doing this sort of thing. So maybe it's time for her to do so.
I agree with Toots. You were originally invited to a family vacation - you, your parents, and sister. now suddenly there are 17 people and only 12 beds (and I would not bet a penny on you getting to keep that room to yourself, especially when there are 5 people (or 3, if the 17 included sister & her BF) without beds. that's a massive change and is not what you signed up for.
but if you don;t go, be absolutely clear with your parents why. Make sure that they know that you were looking forward to vacationing with them
and with your sister, but that you expect to have a room (and bed) to yourself (unless you explicitly agree otherwise in advance) and you expect any changes to the number of people invited, or who is invited, to be discussed in advance.
That way, you are setting clear boundaries for the future.
i would also, in future, be clear when accepting any invite. Ask if it is intended to invite anyone extra, and make it clear that you are accepting on the basis that there won;t be any changes unless these are discussed and agreed in advance.