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?