Hey Kiddo....at some point you are going to have to put your foot down....and that husband of yours needs to support you when you do so.
Sounds like your mother in law is the old dog that needs to be taught some new tricks. I would not let either of my parents treat my wife or my kids the way you are being treated....I;d go shutty on them real quick.
These things do not get better with time. She is using the fact that you are shacking up with her to be petty and unreasonable. Again, stand up for yourself and your husband needs to back your play 100%.