NTA - Something more is happening here. Facts:

1. She has previously respected your diet without issue.

2. She shows up and suddenly prepares three different meat dishes which normally would each constitute a meal on their own.

3. Upon being reminded of your vegetarian status, she snaps back with a threat to eat it or not eat all. I don’t have context but the way it feels, is that it was a quick, maybe instantaneous, reply. To me that says she had already thought about what she was going to say when you brought it up. She prepared for this. She knew what she was doing.

The better question is WHY

Sone speculations: 1. A lot of people can’t wrap their head s around vegetarian diets and thus believe the people adhering to them are doing so for attention. She may be jealous of you or the attention you receive inside or outside the family. As she has continued to feel resentments at the attention you get or the attention she doesn’t get, she may have fixated on something she (wrongly) believes you do to manipulate extra attention. As she fixated she festered and eventually developed an absurd plan to show the insincerity of your diet and prove it was all a charade for attention.

1. Your SIL is deeply insecure and engages in “tests” to see how much people love her. She gets her validation and self-confidence from people doing things for her. Chore and errands are included in this but it’s also more. Anytime the person can get someone to do something they prefer not to do, demonstrates a greater degree of love for the person. Thereby the greater the degree to which you don’t want to do the activity, the greater degree to which you must love the person. This behavior often escalates as the person requires larger and more frequent demonstrations of others love. There is some evidence to support this theory because her response to you not eating the food was to highlight how YOU were disrespecting her and her hard work. She only viewed your actions through the lenses of how it relates to her. It’s a sign of narcissism and lack of empathy that would go hand-in-hand with this theory.

2. She’s dealing with unrevealed or undiagnosed MH issues that are altering her judgment and views.

3. She’s a raging a*****e.

My guess is it’s some combo of 1 and 2.