Usually, domains are renewed in 1-year increments while hosting is only obligated and paid monthly.
Yes, the right thing to do here is keep those active until the new owner takes them over, which probably means paying ahead 1 year of registration and a month (or less) of hosting. Luckily, registration is very cheap. I mean, I don't know what company your in laws are using so I can't quote a price, but, it's the sort of thing where you wouldn't bother figuring out a split and writing it into the legal deal because the minutes asking the lawyer about it would cost more than what you're asking about. But definitely they should check their settings and make sure they only renew the domain for one year and make the hosting bill by month, in case they had ever switched those to longer terms in order to save money in the past.