Welcome to our forum.
I think in most poly families you have to work up a regular schedule, where everyone gets a "fair share" of your time. It can't be as spontaneous as you would perhaps like it to be.
Most poly families have a shared calendar, so everyone knows what's going on.
You'll have to find the way that works best for you, but reading threads on Polyamory.com will give you ideas. Post any thoughts or questions you may have along the way.
Glad to have you aboard.