How to talk to mom about upcoming vacation

I moved states about 5 years ago. My younger sister (now 15) always comes to spend the summer with me. When she was younger, I’d fly to where my parents live, stay there a few days, and fly back with her. Then my mom would come and stay 1-2 weeks and bring her back.

It was great because my mom is a homebody and isn’t much for going out. She might like to go out for a drive here and there, but doesn’t like walking much because she tires easily. My sister wants to do a bunch of activities—ziplining, water parks, beaches, etc—all things my mom doesn’t enjoy and wouldn’t want to do. So we’d do all the things before my mom got here, and then enjoy my mom while she was here.

Last year, my sister was old enough to fly by herself. It wasn’t an idea my mom liked very much but was considering. My mom always ends up buying the tickets like the month before, but it was my understanding she would call me before making the purchase to confirm the dates were ok and if my sister would be able to come ahead. But my mom just went and purchased the tickets for her and my sister to come and leave at the same time.

Now this year, she wants to do the same. She wants to spend the full two weeks with my sister and myself. I would love to see more of her and spend more time with her, especially since she’s been battling cancer for the last 5 years. But, I also want time to have some time just my sister and I and to do things with her.

I want to tell my mom this, but I don’t want her to feel unwanted, like I don’t want to spend time with her, or that I love my sister more. I know we’re not responsible for how other’s feel, but I know she’s going to feel hurt, and that saddens me.

How can I approach this situation in a way that conveys love? How can I process this from the SCS point of view?