If there is a super talented youngster that you can get a hand of, you go ahead and sign him, regardless of who you already have. Wheatley, Biancheri or whoever, if they are going to make it, they will have to beat off competition for places from senior players, let alone a 16 years old.
Also it's not like Obi-Martin is even on the same age group as them, he's one year younger than Biancheri and 2 years younger than Wheatley. We can play both Obi and Wheatley on the U21s, or we can even loan Wheatley out, or put 1 of them on 1st team bench.
With him and the Palace kid as potential additions our FA Youth Cup squad will really be something.