Disabled siblings

I am the middle child of three, my sister is 21, I’m almost 17 and my brother is 12, my sister can look after herself, but she has a hard time moving around so she’s in a wheelchair but my brother has cerebral palsy, he’s blind, can’t talk or walk, both of them have feeding tubes. I’m in a serious relationship and I know he’s the one I want and we’ve been planning our lives together, but I can’t stop thinking about them, most of my friends have/will have nieces or nephews and I can’t stop thinking I’ll never be an aunty, my boyfriend has a younger brother and he said I’ll be an aunty when he has children but It’s not the same, my mother told me that when she dies I’ll have to look after them, which Id love to do I love them so much I would do anything for them, but it really disappoints me knowing they won’t have a normal life and have children or get married ect, I’m so close with my boyfriends little brother and I keep on saying he doesn’t know how lucky he is to have one who can run around and play with, of course I can play with my brother but it’s just hard knowing that he’ll never be able to do that, it really breaks my heart 💔