The girl in my bed.

For context, I have a sister who is 2 years younger than me. She likes to sleep in my bed at night, so I let her. She will sometimes crawl into bed with me and cuddle. She occasionally doesn't even ask. She just jumps into my bed and cuddles me. By now, I should be used to it. She's not a bad kid. But why does it make me feel so weird? When she does this, I feel like I've been invaded, my space invaded. I don't know if I could talk with her about this, but why does it make me feel so weird?