When should I tell my family?

I was wondering when y'all think would be a good time to tell my fiancé's family that I'm pregnant. I'd tell them right away, however, my SIL is days away from having her twins a few weeks early due to preeclampsia, and I don't want it to seem like I'm trying to steal any spotlights or anything (not that that would happen), and I want to give ample time to celebrate her and my nieces and not make her (or anyone else) feel like I'm stepping on toes. Any advice?