idk much about forensics but guess the fingerprints on the letter would be a mess by now - think many in the family read it. Yeah.. may be they can find there aren't any fingerprints of T on it.
I would guess it would be easier to find if the paper is that old... and may be they were smart enough to pick a paper that belonged to T... in case of which there may be fingerprints of T in it. So dont know how successful that's going to be. Can fingerprints last for ~8 years? idk.
Even easier is to compare the words that are used / style of forming sentences. If no one else M can be aware of it.