Advice please

I just found out I’m pregnant. I’m 5-6 weeks. I’m 18, but have been with my boyfriend for years, we live in our own home and pay our own bills. The problem is, my sister (21) is pregnant and her due date was 2 days ago so any day now she’s going to pop. My family is not going to react well, especially being a new baby is literally about to be added to the family. How and when should I tell them? I don’t want to ruin my sisters shine.