CPF interests are earned in the following month of the top up, therefore the lump sum in Jan would let your parent earn interests from Feb to Dec, whereas if you top up monthly from Jan to Dec, the Jan top up earns interests from Feb to Dec, the Feb top up earns interests from Mar to Dec, and so on. If the total top up amount for the 2 cases are the same, lump sum top up in Jan will earn more interests.
CPF interests are earned in the following month of the top up, therefore the lump sum in Jan would let your parent earn interests from Feb to Dec, whereas if you top up monthly from Jan to Dec, the Jan top up earns interests from Feb to Dec, the Feb top up earns interests from Mar to Dec, and so on. If the total top up amount for the 2 cases are the same, lump sum top up in Jan will earn more interests.