I wasnt able to read all the posts. But here is what I have done in the past.
My kids are only allowed to talk on the phone in the same room as I am so I can hear at least one half of the conversation. I never feel bad when I say “no” for any reason. If we do sleepovers the kids do not plan them. I talk to the parents of the friend or relative and plan a good night. I know this is a nephew so you may want to have him more then I would allow one of my girls friends to come over but make a rule…thats a mothers right. 3 over nights a month? or something like that. Only one phone call a day. I would tell the nephew that on the phone and i would also tell him he cant call anymore and that my son will call him tomorrow. I would not hesitate to talk to your nephews parents. I dont know why they would feel offended. just let them know that their son calls all day long, and your son is only allowed to talk once a day so it makes it hard with him calling all the time. They probably dont have a clue and if they do know he is calling you they need to know its not ok.
I hope this doesn’t offend you but it sounds like you need to get a backbone and set some firm limits. As a mother we get put into so many awkward positions and thats why our kids can easily have run of our lives if we dont put a stop to it. Tell your kids your new rules and they will tell the nephew them when he says things like, “ask your Mom if i can come spend the night”
Take back your life! You are not a doormat!