Good responses here. I just wanted to add that I have a problem sometimes telling people exactly what I want, and then I get mad at myself for putting myself in a situation I don't want. You can really spiral downward when you don't assert youself, but I know - I do know - that it can be hard.

On the flipside, you'd be amazed how easy it is to get the things you want if you ask for them. Tell your sister how much some alone time would mean to you, and then take it from there. If you ask me, she would be crazy not to want to spend some time just with her only sibling (assuming it's just the two of you)...but like Capp said she's been through what we would think of as such a surreal situation that her mindset might not be what you'd expect. So I would suggest patience as well.

But don't get mad at yourself for wanting something that's perfectly reasonable, and don't get mad because you have a hard time expressing yourself. You're definitely not alone on that front.