In laws problems

Ok ok let me first say that I adore my mother and sister in law and we've never really had problems. ..until now. My husbands and I got married in October well at our wedding his cousin decided to announce his gfs pregnancy to everyone. I over looked it and now my husband and I are expecting twin girls and we're due three months after his cousin. And my mother and sister in law keep trying to tell me I can't do a certain theme for my girls because it's the same thing as the cousin. This cousin is also talking trash because they think we only got pregnant to steal their thunder. I'm still not saying anything but do I have a right to be pissed here?