According to the terms & conditions of most online casinos, it's not possible to play with several people under one account. But this is in fact nonsense. If your partner, sister or cousin knows your login name and password, he or she can play under your account. It is difficult for casinos to check who is actually behind the computer, and it doesn't matter to them as long as you agree that someone else is playing in your name.
Still, it is better that everyone creates their own account. This way you avoid that people are playing at the same time, which can cause technical issues. And, not the least: every player in your house or family should be responsible for their own playing behavior.
No matter how much you love your children, you will not be happy if they loose hundreds of dollars in your name at the blackjack table. It is therefore wise that you keep your account information private. Your password and login name protect you against misuse, both from strangers as from people you know. Therefore, log out when you have finished playing and do not leave your password lying around.