My DH brother and his girlfriend had a baby in April that they have just arranged a Christening for a date in mid September. We are not particularly close although DH does speak to his bro most weeks and we see them every few weeks when they visit the grandparents who we live a few doors away from so we pop up to say hello (they drive past our house to get to grandparents but would never occur to them call in on us, it's fine we rarely visit them they live an hour plus away but just mentioned to give idea of relationship). The weekend of the Christening happens to coincide with a weekend when I have a long term arrangement to return to my home town for a close friends 40th and to catch up with friends and family who I do not see often as I now live with my DH in the village he was born in. When I have mentioned to MIL we have plans that weekend but it's not essential DH attends with me so he can stay home to attend Christening she has been quite rude and flown off the handle at me this is not unusual she is quite over bearing, controlling, interfering and likes everyone to dance and sing to her tune albeit she has a lot of good qualities and her hearts in the right place. I have messaged my SIL to explain my predicament and have said if she wants me to change my plans I will do as I don't want to upset and offend her but I'm struggling to make an objective decision as MIL has pissed me off so I kind of want to stick to my plans as a fuck you to her interference but I accept this is childish and not fair to BIL and SIL. SIL has said it's my decision and she'll not be offended.
So I guess my question is WWYD would you change your plans for the sake of family harmony and putting family first or would you stick with your original plans as you're not that close, you're not godparent and have made reasonable adjustment for the actual blood relation to attend.