Very annoyed with husband

Mrs.S • Wife, mother, writer... ❤️ a good debate 🗣 Living in the Land of the Free 🇺🇸
So I'd like your thoughts on this. About a month ago I found out that my sister is pregnant with her second child. I'm about 9 - 10 weeks ahead of her. I think it's great and I couldn't be happier for her, as she dealt with a miscarriage early last year. 
My husband has said that she's "stealing our thunder." I know he's half kidding so I just kind of shrug it off, but then last night we found out that she is having a boy (we are too), and he just looked really disappointed and rolled his eyes. He is not happy at all for her and is more acting like a jealous child than a grown man. I mean, what is his problem? This is our first child and I know he's worried that our son will miss out on a lot of attention because my sister's baby will come a couple months later, which I think is completely ludicrous. I don't think it helps that his parents haven't expressed too much excitement regarding my pregnancy. I am just really annoyed with him. Any advice? Am I getting upset over nothing?