Re: WHO will SIT on front seat......
Get a heating element fitted under the front seat where MIL sits and turn it on when she is sitting in it comfortabley next time she will offer that seat to you.
On a serious note it is really a problem for bahoos who live in a joint family system with arrogant MIL's. The only option in this case is to move out , if for some reason it is not possible then you need to be patient when some day MIL , FIL, SIL, BIL anybody realizes this injustice to you and points it out to her.
I know sitting in the front seat with hubby in a car is big issue specially with newly married women. My wife used to make it a point to site in the front seat always when we were newly married and other were also going with us. Even now when we are going somewhere with kids , the kids also want to sit with daddy so sometimes she lets one of them sit in the front sometimes she won't.
On a personal note I do not know what is this fascination of people with sitting in the front seat, I prefer to sit in the back it is more relaxing and less stressful . Specially if you are a driver yourself if you are in the front seat you are doing passive driving. But I have seen grownups and kids fighting to sit in the front seat equally.