It's not weird at all, it's completely and totally normal. I have 2 siblings that are close in age to me from my mother's first marriage (myself and my brother) and her second marriage (our sister) and when she married her fourth husband (who also had two kids of his own from his previous marriage) she decided a few years later that she wanted another baby. I was 17 years old at the time (I'm her oldest) and it made me and my siblings very uncomfortable to think about another sibling being so far apart from us in age. She had a baby anyway (my youngest sister, she's so sweet and I do love her, but I still think it was irresponsible of my mother to have more children) and then 13 months later she found out that she was pregnant with my youngest brother. My youngest brother (I was 19 when he was born, his birthday is two weeks after mine) is not even 3 years older than my DS (about 2 years and 10 months). It's slightly awkward, but I can imagine it would be worse if they were any closer in age.
In other words, what you're feeling is perfectly normal. However, you shouldn't let your mother's choice to have more children interfere with your own plans to have children. I promise that even if your mom does have another child, you will still love your little brother or sister very much. I love both of mine, I became much closer to them than I expected to because I played a big role in helping to raise them. Sure, it might be a little awkward to have a sibling close in age with your own LO, but it also means that your LO is guaranteed to have a playmate within your family to grow up with. I have an uncle that is a year younger than myself and my brother and I played with him all the time, it never felt strange to us that we had an uncle rather than a cousin that was our age. I love the fact that at family gatherings my son will have my brother at the very least (if not my sister too) to play with and become friends with.