In that case, I don't see a problem.
The Probate Office investigates on a case by case basis.
They would be concerned only if something, to use the venacular, seemed off - say, a testator left everything to a charity, cutting off his/her family, or where a previous will left everything to A, B and C and the last left everything to X, Y and Z.
In your case, however, your father's descision to increase your sister's share in his estate in recognition of her services seems logical and reasonable.
Account would also be taken of the fact that family members have no problem with this and would be unlikely to challenge the will.