AIO for being upset at my in-laws over some letters?

Hi yall!! This is my first time posting on here and I don’t use Reddit that much, sorry if there’s grammatical errors, if I ramble too much, or if some details are wrong, I don’t have the best memory!!

Anyways, I live with my soon-to-be in-laws, my fiancé is in basic training for the military and about every two to three weeks we get letters from them. I absolutely cherish these letters with every fiber of my being, I started writing them the minute I got home from seeing my fiancé off in the airport; I haven’t missed a day of writing since!

The first time we received my fiancé’s letters they were all for me, my MIL was furious that she didn’t get one, I brushed this off and tried to be accommodating by reading them out to her and my other in-laws. When I was done reading, my FIL had asked me to tell my fiancé to write their mom, I complied but he kept harping on it with a tone that sounded like he was making it my fault that fiancé didn’t write their mom (I’ll refer to my fiancé as K from now on, fiancé is getting annoying to write). K’s letters is what gave everyone their wake up call to start writing their letters; at this point I was still writing everyday.

The next time we got letters, they were all for me again except for two, MIL was mostly fine and so was FIL; I still read out my letters.

The third set of letters is when K wrote to FIL, MIL, and SIL, I was grateful K wrote to them because when we got the letters I was really anxious that maybe K didn’t write to them again.

This fourth set is when K only wrote to me again, some context, we switched to an app called Sandboxx and we got this set a bit quicker than the others because the letters only take two days to ship and K also gets the same priority shipping.

Anyways, after the third letter I read out loud FIL and MIL started saying stuff like, “Well I guess we’re chopped liver,” “have you asked K to write their mom?” And, “it’s your responsibility to remind K to talk to their mom” literally verbatim said the responsibility thing.

I don’t remember everything they said but FIL, MIL, and K’s grandma all ganged up on me about this. Their tones and attitudes weren’t joking at all, they were genuinely upset at me about these letters. I think this is a bit petty considering the last time they wrote K was May 23rd and that was MIL.

This whole interaction immediately soured my mood so I just read the rest of them, they had more to say after, and went to lay down. I was really happy at first, but now since this has happened so many times I almost don’t want letters from K anymore because they always get pissed about it. I feel like I’ve been accommodating on this issue by letting them read my mail out loud, but apparently that’s not enough and I rarely get a thank you for doing it.

After this happened I tried not to let it get to me but I ended up crying in the shower for a while and I’m kinda alright now. I almost forgot to add this detail but when the letters are majority or all for me, MIL gets short and isn’t as loving with me, I feel like she doesn’t want me around after but she’s fine the next day.

I feel like this is some weird jealousy she has, I really need advice on this. How do I go about this or if I am overreacting, if I do address this with MIL how do I talk about it? I’m someone who HATES conflict and K and I plan on getting married almost immediately when they graduate basic. This has honestly stressed me out quite a bit because I really want to have a good relationship with my in-laws, and it’s not like K and I have only been together for a couple months, it’s been three years. Please give me all the advice you got Reddit!