How will starting a new relationship keep him from being there for you? And is it fair to ask this other woman to wait a month, when she's probably been anticipating this for some time? Will their budding relationship really survive that, or will she take it as a sign that he's not really serious about her?
I'm very sorry about your father, what a hard thing to be going through. You can and should expect him to support you, as much as a long distance partner can. But I have to say, he's allowed to have his own life in the meantime, and I don't think it's ok for you, his married gf, to try, essentially, to keep him monogamous to you... and that's what it seems like this, like you're just intimidated by the idea of him being with someone else.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.