Why? just why...

First off sorry for my language...but damn. So long story short.....we were all eating dinner at the table when I get up to serve myself. When I come back my SIL wipes my babies mouth. Yeah, my 4 month old, so I asked "oh he spit up?" And she smirked and said no then I see it's not spit, BUT mashed potatoes!!! So I asked 3 times "did you give him some??????" And she was like "no, no" then I see that her husband was the one that gave it to him. Like WHY????? He's 4 months, I haven't introduce solids to him nor freaking cereal why would you do that?? He's not the father, he didn't even ask me if I started giving solids. No, all he fucking said was "its fine, it's just mashed potatoes, tell mom nothing is going to happen." And he fucking kept on saying shit like that, "we fed the babies at this age, nothing is going to happen" uh, ok great for you guys, BUT this is not your kid it's MINE. Its not just mashed potatoes by themselves, it had butter,cheese, salt and pepper. Like really? It really bothers the shit out of me because he didn't even ask, like he just went ahead. My blood was boiling, so I just couldn't eat anymore. Way to go dude. Yeah yeah I know nothing can happen with a little, but I don't care, you just don't do that with someone else's baby!!!!! You just DONT. God this happened last night, and I haven't talked to my SO about it, so I just haven't let go. I never did that with their kids, nor will i ever do that to anyone's kids. If they started solids at 4 months good for them, but I don't plan to. Now I feel every time we go over, I'm going to be like a hawk just staring at the baby down hoping he won't give him anything. Like not even my husband, dude. Ahhhhhh I just can't right now😤