She's being a bitch.
There's no such thing as "seeing your family too much." If she refuses to see your family for no reason other than "we already saw them this month" it pretty much means that she doesn't WANT TO for some reason or another... i.e. she doesn't like your family or something, which isn't fair to you if your family has always been nice to her.
Don't listen to the irrationality or "time warp from 1950" shit. If married life were still as it were in 1950 then marriage & fidelity would probably be the norm as opposed to the exception.