Advice on telling a certain family member...?

You stop worrying how unreasonable people will react, just tell them like you would other family members and if they are "furious" say this, "it is really sad how you are angry over such a joyous occasion. MY life doesn't stop because of your conceptual problems and the world doesn't revolve around you and your uterus" and end it at that. Let them steam and be furious, but you shouldn't be tip toeing around people.