While there are in-laws who are just plain nasty and evil, some may have intentions that are pure and noble. And what you sensed as their meddling in your affairs may just be their equally noble attempts at being motherly or fatherly. And that bitchy sister-in-law of yours? She must just be behaving like a real sister who happens to be just, well, bitchy. It will help to remind yourself that you also had spats with your own parents and siblings. How much more with these folks who were total strangers before you met your husband or wife?
Someday, you will become a mother-in-law or father in-law yourself. And your first instincts will always be to protect your offspring and the offspring’s children. Your intentions, good or bad, may be correctly perceived or misunderstood. Whatever, the case, try to communicate as well as you can, and hope for the best.
Me? I have a 9-year old girl, and I think I already have plans on how to terrorize would-be suitors 10 years or so from now.