Have they not been around when the 2 of you are together? Slowly incorporate yourself into their daily lives. A dinner here...a movie there (together). Eventually they will realize that you are there to stay. Give them time to accept each step. Springing it on them all at once could doom the relationship.
I bet your just apprehensive and they will be perhaps surprised or not,.. but they are already fond of you.
It's thei fathers job to tell them about the relationship, not yours. I'm a widower and I know what I'm talking about. He should be the one to carry the message; they'll have questions you can't answer and they won't want to ask you.
Not until he's done. If they have questions for you, simply answer them.
Im curious first how long ago did she pass?