Sparkly your post sounds exacly like my life a few months ago (DS is 18 months now and is making a lot of sleep progress, so hang in there).
My TW is my sister's boyfriend. I'm really not fond of him (there are a LOT of reasons for this but I won't get into all that). He has zero respect for my rules when it comes to DS and thinks he can just ignore me and do what he wants. Today we were over for dinner and we were just waiting for it to finish cooking. For some reason he decided this was a good time to take out this massive chocolate chip cookie (like bigger than your hand). He proceeds to offer DS some. I say no, he can have some after supper but not before. He goes ahead and gives some to DS. I speak to my sister, she reluctantly tells him not to give him anymore. Next thing I know he is offering DS another piece. Again I tell him no, and again he ignores me and breaks of a chunk which he hands to DS. I really had to restrain myself (this is not the first time he has done something like this). I of course had to be the bad guy and take the cookie from DS and then we left (without dinner). My sister is pissed (at me, not him). Poor DS ended up having to wait to eat an extra hour past our regular dinner time, which resulted in about 30 minutes of tantruming, and I alternate between being livid and feeling guilty.
Your sister's BF can eat a bag of dicks. There is zero reason for someone to just ignore another adult child related or not.
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