I think it's a classic case of miscommunication.
[still laughing at under13's recommendation- dude, I know what your favorite recommendation for anyone is,"leave the church
"]
So you are busy playing in Ab then your papa comes and plays in Db..
The solution to this is to tell him at home [if you stay with him] that dad, whenever I am playing and then you come and play on a different key, you throw me and the whole congregation off. Next time before you start playing, please ask me what key I'm on. Alternatively, you should tell me what key you want to go to before you start playing.
I understand your frustration because my pastor sometimes does that. But I think at some stage he picked up that I was not impressed, so he tends to ask me what key I'm in [rare] or tells me to go to the key he wants to play in. At least that is respectful.
Half the time though, I kinda have to figure out what key he is in, but it's not too difficult coz I can see his fingers from an angle.
As for the others [who "can't play", depending on your familiarity with them, you can either ask them politely not to play as they throw you off, or ask your dad to talk on your behalf, without exposing you