Hers but not ours?

So my boyfriend and I have been officially together for 2 and a bit more than half two years but have been on and off for the last 7 years. I’m newly pregnant like 6 weeks along. He knows but accepts but isn’t happy my decision to keep the baby because I think we are more than ready just because it wasn’t planned. Except his sister (who is his least favorite person) is about a month ahead of me pregnancy wise and without giving it a second though he was willing to take that kid in because he could give it a better life if his sister couldn’t keep her life together. I’m definitely hurt by this. And he’s accepted my  decision but he’s not excited and I’m having a hard time trying not to take it personally. Is there anyway that it’s not because it’s me?? Any insight and advice is greatly appreciated.