Oatmeal in bottle

My son just turned four months old and I feed him nothing but formula. Well we are staying with my MIL until we find a home to buy. My MIL always insist on trying to feed my son real food but I told her it isn't a good idea. Tonight she put oatmeal in his bottle even after I told her it wasn't a good idea and he kept choking on it. I'm so tired of this and I don't know what to do. I tell her all the time but she'll do it anyways. What can I do to stop her from doing this? I've told her several times that I am not feeding him food and tell her no all the time but she always takes the bottle and says "it'll be okay.". I'm not sure how much longer we will be here but I'm scared he could choke...to death if she keeps feeding him food. 
***This wasn't baby oatmeal it was normal oatmeal.
***I am the one always to feed him but she always sees me and tells me to put something in his bottle and will keep insisting I give her the bottle over and over again and if I tell her no she's pissed and will argue and cuss.
***Another thing is that my husband does tell her no and she tells him to shut up. I try my hardest to not let her have his bottles but she won't stop. She gets so angry when we tell her no. 
***I am quick to anger but the last thing on my mind is getting into a fight with her (for the person that said I should beat her ass). I feel that's way to far.