I don't understand this.
How should I help my family, and why?
And in what way am I using/would be using them?
Some people don't understand. Me coming out to my father and then continuing my own "great life" would be very egoistic. I would leave lot of debris behind. I don't want that.
No one should be forced to come out for any reason.
But you're afraid/unwilling to even tell your father that you don't want to go on this date.
What would be the consequence if you declined to meet her? Is there a chance he'll be violent and abuse your other family members? Because a lot of replies in this thread are operating under the presumption that there's a risk of violence. If that's not the case, then clarify, otherwise your deep-seated, primal fear of disappointing your father is being excessively coddled here.
If telling your dad that you don't want to meet this woman will lead to him being angry - but not violent nor abusive - then you have no excuse to involve this woman into this situation. Respect yourself, your boyfriend, and this woman, and decline to go out with her.
As for how and why you would help your family: if there's a risk of violence in the home, you help remove them from the vicinity of the violent actor. Why should be obvious.
If just the thought of their living with an unhappy (but not abusive) father is overwhelming for you, then send them on a "vacation" to a nearby hotel or have them visit you for a while.