Not much of a background, but we have generally got on OK, we are very different people, mums etc and naturally have different opinions on a lot of things so will never be friends but have generally always been civil. My DH never used to get on with her at all, but these days they seem OK with each other.
So this week we've had a big fallout cumilating today at my PILs home. During the week we had to cancel the gathering at my in laws this weekend as one bil tested covid positive and he and his family are now isolating and another BIL also self isolating due to being a close contact. As the arrangement was cancelled, they suggested other dates. We were on holiday this week but said we couldn't do the preferred date. We were asked if we could move it, and I said no as it was my planned birthday celebrations with my family and friends. She asked if I could re arrange with my family, said sorry but due to family holidays etc no. I said we would check other dates when we get home, never got a response but assumed that meant it was OK and they would wait to hear from us on other dates. Yesterday she messaged my DH asking again if we would move my birthday celebrations as it was so unfair my PILs would have to wait so long for all the family being able to get together. My DH messaged no and said we would cross check some other dates and let them know. This message really annoyed me and my DH and I had a big argument because I felt it was very rude etc.
Today we turned up at my PIL house to see them give presents etc. Expected it to be just us but found out SIL and her husband and children were all their (her husband is currently in self isolation, day 8 out of 10) so my husband and I said we would stay outside (as I'm CEV and know I haven't got antibodies from the vaccines, I didn't want to go inside near someone who is meant to be self isolating) anyway my SIL came to say hello, she asked how I was, I said fine thank you and you. She then asked what I was doing for my birthday and whether I had considered splitting the celebration and seeing some family another day for my birthday as it had really upset my MIL that I was preventing the whole family getting together. I overreacted and said you've asked too many times and no I will not change my birthday celebrations for her. I'm fed up of her asking. Yes I l was rude in my response. She stormed into my IL slammed the kitchen door and my MIL came out saying she didn't want me to be around if I was going to sit outside and upset SIL. I admit I overreacted but hubby drove me home and then returned with the kids. now I feel awful about upsetting my MIL and have messaged her and my FIL to apologise but they say I have to make it up to SIL as well but I will be honest I don't know how to as I still feel she was unreasonable to ask again about me changing my plans. Therefore how do I be the bigger person and try to clear the air with her? Just no idea how to approach it