Fiancé disrespecting mom?

I’ve been with my fiancé for 4 years engaged for 2 years. We’re waiting to get married so we can work on our debt & other things but our relationship has been good so far. He does get along with my family well & there’s never been an issue until New Year’s day. My mom lives about 2 hrs away from me & stayed at my cousin’s place for the holidays since it’s bigger & I was on my way over with my fiancé to say bye to her before she left. She had asked if I could get her & the family (total 5 people) Starbucks that she would pay me back & I agreed. I did get a little annoyed at having to stop on my way but it wasn’t a big deal. We got to my cousin’s place & my mom said she would pay me back & I said no because it was only $20... not a big deal. I guess my fiancé looked bothered & my mom asked him what was wrong to which he replied “I feel like you guys take advantage of her like this & it isn’t fair that you guys won’t even pay her back for it” & I turned & asked him why he was talking to her like that & everyone just got quiet. My mom actually got up & left 5 min later. I stayed at my cousin’s the rest of the day as my fiancé left. I just felt that that was very disrespectful of him to say, especially over something so minor as coffee & also the fact that I paid for it doesn’t give him any reason to say what he did.

My family never asks me for money, ever. For the holidays, they always cook the dinner & I ask if they need me to bring anything because it’s only fair. My fiancé’s brother is constantly asking him for money just because he’s irresponsible with his & I’ve told my fiancé my opinion on it so I guess that’s where his comment stemmed from. But our relationship has changed because of this & I feel very hurt by what he said & how he acted towards my mom. He did call my mom & apologized to her but I’m scared to even bring him around my family anymore. Am I overreacting or should I be looking into this more closely?