I'd find it strange I suppose (especially given that I had my OWN child when I was 21, so if my mother had one the year prior, my sibling and my daughter would be only a year between them in age).....but if that's what she wanted to do, then it wouldn't have been any of my business, I'd have just accepted it and that's that. After all, I moved out 4 days before my 19th birthday, so it's not as though the baby would have been much to do with me really.
Course my mother was 54 when I was 20 lol, so that obviously was never even a possibility, but as a hypothetical then no I wouldn't have minded.