Am I over reacting?!?

Vick
So long story short. My due date is 20th Aug and my mother-in-law is due to get married on the 19th! She wants my OH to use his car and drive her to the church then to the venue of the reception leaving me like a sweaty beach whale and his 6 year old son to fend for ourselves and he doesn't see a problem. I'm literally in rage. 
I have said we can't promise anything as anything could be happening at that point so close to the due date but OH days "oh but it is my mum" despite him not even being happy about the marriage (another story lol). 
Have tried to discuss it with him and explain my concerns but just gets brushed off so just lost it and said to just tell her I won't be there! Whoops but I was mad. 
His mum has plenty of
Capital having been in round the world trip, new car, extensions to the house etc to hire a car! 
Would appreciate your thoughts as I'm going stir crazy x