As for the family...You can still be friends with the in-laws. In my oh so strange world....believe it or not, I talk with the ex-in-laws way more than they do with their daughter. My MIL just called yesterday and we talked for an hour....about the kids, business, my bum knee, all kinds of stuff. That said I have one hard and fast rule that I explained to her mother, father, and step father (actually this rule applies to my family also)...I don't nor do I accept them talking about what has transpired...no bad talking, prying, anything related to the ex. They all respect that, though the step mother does try to get the occasional barb in there.
For me it is easy...we can talk all day about the kids and I (I even often ask for parenting advice), but the ex is the past. Last summer the kids and I went to Disney with my sister's family and parents. At one point my mom started bad mouthing the ex in front of the kids. I quickly pulled her aside and told her to stop or we would leave. No matter what my ex has done....I will not have people putting her down in front of the kids....and that goes for me too. The kids will form their own opinion of their mom themselves...not by other people.
I agree with Melissa's question....I am curious about that boundary as it appears very weak and transparent at times.