Did she have a prior will?
In about 25 states holographic wills suffice. A holographic will is a handwritten will. In order to be legal and enforcable it must be completely in the handwriting of the author with no typewritten parts; must be free from edits from any outside party; must be dated; and must be signed by the author. Holographic wills need not be notarized, as is the case for typewritten wills.
However, even if your state does provide for the validity of holographic wills, there is the question of whether the letter was intended to be a revocation of the other will (if one existed) and whether it is sufficient to act as a will. My guess is that it was not sufficient in detail to act as a will anyway, but I don't know the content of your letter.
More importantly, for any will to be valid (type-written or hand written) the testator must be making the will of their own volition, must be of sound mind, and must be free from undue influence.
No one in your family may even try to assert that the letter should act as a will. If there is a will contest (law suit questioning the will) then the issues of whether she was of sound mind and whether the letter can stand as a holographic will (if you are even in a state that recognizes holographic wills) will most likely be the issues addressed by the court.
Does she have significant assets so that this is a worthy issue to even litigate?
HTH.