This is my first day as a forum member so if I break any unwritten rules apologies in advance. 
I grew up in a huge family/village environment and its perfectly normal to walk in and out of each others doors, anybody who wants to feel an extra security just gives out keys.
The rule is if an outer door is open or ajar frrl free to come right in. If its shut, knock first. If its locked, we're up to something that's nobody else's business so only knock in a genuine emergency...
That said, I see your husband's point. In-laws are given full family rights so if that's the current status carry on as normal. If he weed all over that gesture then he gets downgraded to a knocker. If you're maintain good terms for the family harmony you and husband should consult the rest of the family.
It took me years to convince my son in law to just walk in, he causes mild worry insisting on knocking. Good luck.