To Xmas or not

I tried to make plans with my grandparents this year but they wouldn't make plans with me so I planned for a Christmas alone - happens most years but I enjoy it that way. Though some years they'll do this, say they don't want to make plans and then a day or two before Xmas they'll ring saying they expect to see me boxing day. It ruins my plans all the time! I don't have a freezer so I of course buy fresh food to last over the Xmas period as well as any other things I've planned for myself all goes down the drain when I then feel obligated to go to them last minute. Every time I do this, I leave their house wishing I'd not bothered and treated myself to the day I had planned. So this year I was prepared - either they book me in the diary so to speak for Xmas, or I will make my own plans and won't be having them ruined because someone else changed their mind. I think that's such a selfish thing to do, not to mention rude. This year they didn't want to see me or my sister(15) but that was fine with us, I had already agreed to take her boxing day for a few days and we'd decided what we wanted to eat etc. I was just about to buy the last few bits we needed when I received a message from my sister saying they had spoken to Nanny and she wanted us to go theirs boxing day...🤬🤬🤬🤬🤬....

So now once again MY plans are ruined. I've said I'm not going and that this is exactly what I've been talking about happening for the last few months. But now my sister is going there for half 3 - meaning I'm assuming nanny will be cooking for her, so I won't have that Christmas meal with her myself like we'd planned (and I was really looking forward to) and now there's too much food to eat in a short space of time as well as the fact that I never needed to go through the effort and stress of getting it in to begin with! - although she can't even tell me for definite if she's even eating there or not! Am I wrong for feeling angry, annoyed, hurt and so much more? I haven't spoken to my grandparents about it yet, and of course I don't want to upset them but I've allowed this and so much more my whole life without really saying anything so to protect everyone else while I'm the one that suffers. Nanny didn't even phone me, she phoned my sister (who's a child!) and made arrangements behind my back when she knew the plan was that I was having my sister on boxing day months ago when I originally tried to make plans. So she's really thrown a spanner in the works. What would you do in this situation? She now apparently wants me to give her a time when they can drop presents for me off to my house but again I'd made plans and don't have a time that I can just wait around for them. I knew this would happen, spoke to my therapist about it and so made sure to avoid this by trying to make a plan with the grandparents but they didn't want to make plans even though they were happy to schedule the rest of the family in and now once again I'm the after thought expected to jump through hoops and I'm not about it any more!