Its a very personal decision. My brother in law is gay and came out to the super religious grandfather. Grandfather not only accepted it but flew out to their wedding. I think if anything it brought them closer.

I myself announced my bisexuality on Facebook because that's just how I am. It's very freeing to have people accept you for who you are, entirely.

If it is important to you to let them know this, tell them. Especially since you say they will love you no matter what. I wish you luck in your decision.