We only loan equipment out to friends we trust and 9 times out of 10 it comes back in better shape then it left. My neighbor insist on washing all my tractors before he brings them back. He has to have close to a ton of my mud on his property as my tractors don't get washed very often (it would be a full time job with the mud we have here). He also changed all the fluids in my 1920 before he returned it and changed the filters.
There is only one person who borrows our corn planter every year. I regret ever letting him take it the first time as now it is hard to say no. He hauls it 15 miles away, keeps it for a month (sometimes longer) then brings it back dirty. Of course he never takes it just after we are done using it, he waits till we get it all cleaned, oiled and greased for storage before he wants it
I have found a couple other used planters that would suit his needs but he doesn't want to buy one (and why should he when he can use ours for free). His ground is very rocky so things usually come back slightly damaged. It is going to stop soon though, hopefully this is the last year as I need to fix my planter up and I am not going to be loaning it out once I do. I will look for some more used ones and tell him about them so it would be up to him what to do.
As for your family matter, you can suck it up and let them use it and abuse it to keep the peace or you can just give them the number to the rental yard and let them decide. If this silly thing is enough for them to not want to let you see the grand kids then I feel bad for their kids. My youngest sister moved to New York City with her boyfriend and son. She refuses to talk to our step mother because she didn't have the time one day to do her boyfriends laundry. So when she moved she will not tell us her address and has nothing to do with my father and step mother (she sends my father cards still but with no return addy). My parents are not happy with the way she acted and now concentrate on the grand kids they do get to see. Best of luck to you, I wish I could tell you what to do but you will have to make the choice and live with the decision. Take care.