I can't give a definitive answer but, in my own experience anyway, it should be fine for multiple family members to have an account.
For almost two years I shared an apartment and an internet connection with my sister PaddedPaws, who's also a member here. We both delurked and signed up at the same time, the same day I think. If that weren't suspicious enough, we both posted in the same introduction thread. We've never had any problems.
When it's multiple members of the same household I think it's pretty obvious. They usually have their own style, post independently of each other on different threads, etc.