So now you're saying you want:
1) that you know when your family is online.
2) that your family knows you're online.
(which is again different from both previous things you've said
)
In that case, why don't you and your family simply set a special notification sound for when you or your family comes online (and even goes back offline or any other status for that matter) together with using the (always on-top) desktop contacts?
(I know I do it that way for those "special" friends of mine, and I always know when they are online or not, no matter how many windows I have opened or whatever I'm doing on the PC 
)
Just opening a new chat window, in this last case you've described, just to see if someone is online is not a good solution. eg: it clutters up your task bar with an otherwise useless window, if they go back offline you wont notice it, etc...
/ 
