As the previous posters have said, your sister was out of line. however, I can in a way, see things from her point of view.
I am one of four girls and as much as we love each other, there is always been a very feint, underlying competitive streak amongst us. Now that we are all in our 20s, it has all but disappeared.
it is a fairly different situation, but my second youngest sister did get pregnant straight out of school at 18. i’ll admit, I wasn’t entirely happy when she announced she was first pregnant. my disappointment didn’t have anything to do with competition, I just really feared for her where her life was heading at the time. Her partner or her didn’t have secure jobs or educations, and I worried about the life they had to offer a child. He was fairly wild with a very shady past, and between them they didn’t have $10 to their names. (My parents came to the rescue in the end, setting him on the right path with a secure job in their business, and all is going well for them now).
I didn’t actually say anything to my sister at the time, I didn’t need to. She knew I wasn’t 100% impressed with the situation, but I made a real effort to get over my own feelings which were irrelevant in the scheme of things, and to just support her.
But, I will admit I did have tinges of jealousy. I am the eldest, yet my younger sister was the first one to have a child, being my parents first grandchild and my sisters first niece or nephew. Then when she had a boy, the fact that she’d broken the female drought of my family also had me feeling tinges of jealousy.
But I think the worst of my own feelings were bought about by the fact that I was in a crap relationship. I was jealous of the fact that she had a baby and I didn’t feel like I was in a position where that could be a reality for me. I knew I wanted children, but I couldn’t see myself doing that with my partner of 10 years. That was a really horrible feeling for me. It really hurt me to see happy people together planning families with both partners equally on board. So I was jealous, I was angry at myself for not leaving, and it was just a crappy situation all-round for me. Could that be something your sister is feeling?
oh, and congratulations and good luck! 🙂