Sister in Law INSISTS on being maid of honour

I've read posts where the sister in law has wanted to be a bridesmaid but not one where she INSISTS on being maid of honour. This has never been discussed with me and my mother in law just announced (in front of the whole family) that L already bought a dress as maid of honour. I was completely stunned by this revelation. L and I are not even close, and to nominate herself as MOH and buy a dress without asking me first is just SO, unbelievably RUDE? It gets worse. I then find out my mother in law had asked the cousins to be bridesmaids and they also already bought matching dresses. I asked if my friends could get involved but they (both MIL and SIL) said something along the lines: 'We already bought matching dresses. Let your friends be ushers instead.' SIL had also insisted her boyfriend sit at the top table with us (mind you, in place of my family member)

I had planned for years to have my best friend as MOH but I can't even have her as a bridesmaid now? MOH also wanted the throw me a 'hen do' but invited all her OWN friends along (Yes, people I didn't know!) They were going to rent an apartment, but if my friends wanted to come to MY 'hen do' they had to find a place elsewhere - I didn't go because it was so ridiculous!

This is incredibly distressing and I don't know what to do. On one hand, SIL is very helpful because she's a makeup artist and in charge of all the floral arrangements, as well. On the other, I won't be 'happy' having her as my MOH because my best friend won't be at my side. I also don't want to ruffle feathers because the family is very kind to me, bar this 'insistance'. I genuinely think they are behaving this way as this is the first wedding in their family.