Pregnancy Announcement Issue

So I'm wondering if it's rude to announce our pregnancy at a low key birthday lunch my parents are hosting for my sister in law. It's right around the time we were thinking of announcing and it'd be nice to tell parents and siblings at the same time and in person. Otherwise, we would have to arrange another get together soon afterwards, which can be hard to coordinate with everyone. My sister in law and I are the same age and we are always talking babies, she just had her first a few months ago. So I know she would be thrilled to hear the news, I just don't want to take away from her day at all. What do you all think???