This makes perfect sense. After all, you planned this cruise and are looking forward to enjoying it with the people you chose to cruise with. I think it's unfair for your mom to want to 'muscle in' on that, particularly if she hasn't discussed it in depth with you first. She may say she doesn't want to infringe, but to me it seems as if she already has.
I can really understand you don't want to do everything together. Twice we took my dad with us to WDW and in the planning he insisted he would want to spend some time on his own, either when he needed to rest or if we were going places he wasn't interested in. In reality, he insisted on coming with us everywhere and didn't give us a minute to ourselves. He would traipse round the shops like a petulant toddler so that we gave up and went back to the hotel. And on our wedding anniversary he wouldn't let us go to dinner on our own, but then refused to eat in the restaurant we'd planned, and the next 3 we suggested, and we ended up in the food court at POR! The first time we took him (just after my mum died) we tried to give him the benefit of the doubt, but said we'd never do it again. 10 years later we relented and took him again, thinking from what he said when we were planning that it would be better, and it was worse!
I don't want to get flamed here because I know many people are very close to their parents, but families are complicated things, and for me the idea of my dad being on a cruise with me would pretty much be my worst nightmare.
Yet I would've loved the opportunity to cruise with my mum, particularly on
DCL as she loved WDW.
We had talked of the 4 of us going to WDW together, but she died before we had the chance.
Perhaps you can speak to your sister and discuss strategy with her. And if you don't ask for staterooms or dining to be linked, then maybe that will help give you your own space.