My theory. Uncle knows everything about what's going on with Antonio. He doesn't trust Antonio anyway because how hot headed and impulsive he is. On top of that if it were to come out that the next in line isn't a straight man, it would probably be very problematic for the family. That's why he's training Gracie. And once he thinks Gracie is ready to take over, he'll get rid of Antonio so that Gracie is next in line because Luna wouldn't want to be the next in line anyway.
The problem I have with that theory is, Cordia has been, and is outright against Gracie getting involved (in the Family business).
Whenever Wilfred tries to raise even a point that "Gracie is intelligent and could be a valuable asset to the Family" etc she shuts him down (that in one case in game results in exasperation from Wilfred).
I mean yes, MC will help Gracie to convince Cordia so Gracie can now work for the Family in the upcoming update, but Wilfred has been teaching Gracie for a long time before even MC arrived at DeLucas' gates in the game's beginning.
So I just find it difficult to believe Wilfred would be able to forsee all that and plan way ahead of time. Sure, he would know all this time about Antonio's short temper and probably even about him not being straight (bi/gay), but that I don't think that means it's a sure thing for Gracie to be in next in line, especially considering how strong Cordia feels about the matter.
It's a bit different now with MC in the mix, but to me it feels more like he's been training Gracie with a little bit of hope that maybe, just maybe somewhere along the line Cordia may let her daughter work for the Family in some capacity. But what he probably didn't account for is how MC being there had effect on Gracie (and rest of the Family for that matter).
Probably he's keeping on eye on MC's interactions with the girls with the mindset "This should be interesting. Let's see how this turns out."
Also, if I was an actual soldier in DeLucas, I think neither Luna nor Gracie should be the next head of the Family, heh.
Sure, Luna on the outward appearance looks like the more unstable one due to how brash she might get (especially when she gets in the fights) so that's likely a bad choice, but I feel Gracie is also pretty unstable in her own way (her completely nonchalant way of reacting to Trino's death, and almost Jekyll & Hyde like reaction to MC after Blizzard Princess event [despite her saying it's a joke]).
Antonio's short temper will cause him problems should he become Don, but I don't think the girls are the right candidates to be a Donna either, with each of them having their own flaws (regarding their capacity as a Don/Donna).
Family's heir in my opinion needs someone who is a bit more level headed; MC could be a good fit if he stopped being a wimp/coward (whichever term that applies to his current state
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) and be fully committed to the Family.