The Bayern midfielder's current deal expires in 2015, and it has been speculated that the German international will seek pastures new after talks stalled over a new contract.
Such reports have been denied by his club, but Felix has revealed that talks have taken place between the two siblings regarding the Premier League champions.
He told Sky Germany: "We have already talked about [Toni potentially moving to Manchester United].
"Manchester has been the team of my dreams since I was a child. However, I don't give him any advice.
"He will take a good decision for himself. There are worse situations than having to decide something like that."
United are expected to have a summer budget in the region of £100m.