Sister in law over stepping boundaries

Me and my man are having our third I haven’t told anyone yet I’m 13 weeks I got pregnant really fast after my second who’s 22 weeks I took the birth control shot but still ended up pregnant I can afford my kids including the third. My sister in law found out from my man and was like oh did I tell my mom yet and he said no I haven’t I want to tell everyone at my own time because she told everyone without my consent with my second kid and I was pissed it’s my pregnancy I should be the one to announce I told my husband he shouldn’t of even told her because she got a big mouth and I hate that . So she only spoke on the phone with my mom twice they never met in person I know they are in laws and should of met from before we had kids when we first became serious and my parents tried to invite her and her mom over they even made a dinner but they never made the effort to come and she said if I don’t tell my mom now she is I’m so pissed I want to tell her off but I don’t want to start something what should I do because I know now if I do confront her which she hides every time she’s causes problems with people and won’t answer her phone or door I will blow up and that’s the last thing I’m trying to do it’s childish and I don’t want to go to her level we are all in our 20s and should act like afults and I don’t want trouble .