Hey Danny. Sorry to hear about your grandmother; my grandfather also passed away just last month after a comparable time in long-standing pain. It's for the better; it's good to hear that you know that
As for the kids.... I wouldn't in the least call what they do "minor". I'm only 18, and while I'm hardly an exemplary person, man would I never, ever consider destroying a relatives property or disrespecting others to the degree these kids are doing. My suggestion would be to get locks on all your doors in your house to important areas (ie your computer room, bedroom, gaming room, etc) and keeping the keys on your person at all times. If the kids misbehave, give them an earful. If the parents complain, well it's YOUR house so it's YOUR rules, and they aren't even being civil, nevermind following any rules.
I'm personally a victim of childhood physical abuse from my father, so I'd never condone a spanking, or the like; what I would try to do is assert your **AUTHORITY** where ever possible, and making sure that these kids are staying in line.
OTOH, ensure you are being compassionate as well. You don't want to be the Bad Guy; try talking with them and getting to know them. If they aren't respecting your property, they liely dont respect you - so turn that around. In any case, an open line of communication is a great tool which I find isn't utilized enough.
Good luck
cheers,
--N