While it’s clear that there are many hurt feelings about what happened after his original interment, I think it’s time to forgive your sister.
It’s a fact of life that sometimes unforeseen circumstances prevent people with the best of intentions from keeping promises they make. Forgive her if you can, but don’t ask her for money. She may owe you an apology, but she does not owe you any money.
While the power strip you installed is the practical solution, the problem is that the people you have described are thoughtless. Polite guests leave their host’s home in the same condition it was in when the guests arrived. In this instance, it means being considerate enough to plug in any appliances that the guests unplugged, so the host doesn’t have to wonder why the darned thing isn’t working after everyone has departed.
And as for electronic items foolishly left on a wet counter, if there was any damage, the fault would not be yours. Your niece/nephew was lucky.