"Do you deceive your father?"
Translation:Voi îl înșelați pe tatăl vostru?
Not necessarily, though it does sound a bit weird. You talk to a person. A group of people, including that person, deceives the person's father. He is only the father of the one you are talking to, not of the other people in that group. So, the first 'you' would refer to a group of people(the ones doing the deceiving, whatever that means) and the 'your' would refer to only one person(the one you're speaking with, who's also part of that group).
I believe that this may work in practice while you look at different people one after the other, but grammarwise it still seems wrong to me: the person addressed in a speech should not change mid-sentence. Maybe for English natives that doesn't matter because it is the same word "you", and you have this politically correct he/she = they thing. But thinking German, French, Russian... it just sounds like you couldn't follow through with your, I forgot, how shall I finish this, any questions? :-)
You can work as "tu" and as "voi", so what is wrong with "Iti inseli tatal?"
Several things are wrong, I guess - but indeed, you will address "tu" with "înșeli" as you did. But like in English, "deceive someone" is a transitive verb that needs a direct object "someone" (=accusative) and not an indirect dative like "to someone" when talking about the one who is deceived.
Anyway, your "Îți" is dative of "tu", meaning "you deceive to you", but you want accusative of "he" = "Îl" (the father who is deceived), so it must be "Îl înșeli".
And then, apparently - as I gathered here - Romanian doesn't accept persons (tătal) as accusative object required by a transitive verb (a înșela), but refers only indirectly to persons by using the prepostion "pe" (= by, on) - I would like to learn why this is so myself!
But because "tu înșeli pe tătal" is still missing a direct (accusative) object for the verb you need the extra "îl":
(Tu) îl inșeli pe tătal.
PS. An intransitive version of the verb would not need a direct=accusative object, like in "You often deceive" - but that is not possible with all verbs. For example, "You often accuse" sounds incomplete without an accusative object, and a dative "accuse to so." won't work here, whereas the intransitive usage in "complain to so." is ok, because "You often complain" already is a complete sentence.