I was 18 when my mom had my brother 3 years ago. He was born when I had just started college so I was really sad about only being able to see him every few months, but now I've graduated and moved back home and me and my brother are closer than ever now I'm starting grad school this summer so I'm just enjoying my time with him while I can.
I think my Mom punched out her tubes after my little sister, plus she's 47 soooo yeah, it would be highly unlikely; however, I'd be happy for her if it somehow did happen. I think my relationship with a third sibling at my age would feel more like an aunt/niece or nephew than big sister/little bro or sis.
I mean she turned 45 last month but idk...
Sometimes I say that I'm not taking care of this one (I have a 10yo bro) but then the other part of me would like to take care of it now that I'm older