It's fine for family members to play CS, too. You may get an alert reminding you about the one-account rule when you go to trade with them, but just sharing the same IP address as another user is not going get you in trouble. Many users play CS on public wifi that any number of other users could be sharing after all, like at school with their friends!
Just be aware that it is considered cheating if one account is serving to benefit the other, like if your husband were to regularly adopt pets to send them to you. This applies even if the other account is controlled by someone else. Both of you should be playing CS for yourself.
It's not required to avoid trading altogether, but if you want to make sure you don't get accused of cheating, keep your trades fair with friends and family. Make sure that the pets you trade are fairly close in value to the pets you are receiving so it doesn't look like one of you is getting all the good stuff in return for trash, and don't allow someone to constantly give you free stuff if you never give them anything in return.
If you ever want to make an exception, eg. sending a birthday gift, you should get permission from an admin first by sending a
help ticket as your accounts may get flagged otherwise. And if he signs up and decides CS is not for him, he should to send his pets to the Pound instead of sending them to you. It would be considered cheating if he signed up and then gave you everything he could adopt.