Well, first of all I think you are putting the cart before the horse. You are only just starting this journey and there is far more to think about than what family think... you have a world of things to sort out. The more you are in it the more there will be. Baby steps my dear... baby steps
I think it might not be wise to consider that you will be with her also. Its a lovely thing to love a metamour, but to love them "that way" doesn't often work. It doesn't sound like right off the bat it was about the three of you in a tiad, so let them alone and enjoy your newly revived partner.
You sound like you have empathy NRE for your partner and his girlfriend. That is lovely, but keeping it in perspective might be a good idea... someone has to anchor you all to the ground.
Give this all a chance, it isn't all glorious and lovely... if you have some jealousy then best to address it and get more settled before telling family. It becomes far more complicated and intense once others know. All those little doubts become HUGE. Make sure there are none before hand...
Mono was in our lives for 8 months before we told family. We told them because they forced us to tell them. It was far too early. We had just worked out boundaries about time together on a regular basis and were getting comfortable with the dynamic... you can read back and see.
It's been almost two years now and he lives with us. It was a hell of a ride but we did it only because we took our time... there is really no rush to do anything as if you love someone there is often that feeling of wanting to spend the rest of our lives with them. Riding on that has been helpful to us.