What is the yearly limit that you can do cash top up to family member's cpf?

$7000 SGD per year of assessment is the limit

The maximum CPF Cashtop-up is $14,000 (maximum $7,000 for self, and maximum $7,000 for family members).

The annual limit for mandatory + voluntary contributions is $37,740. So the max you can top up for your family should be that amount minus any mandatory contributions they had.

That said most people top up for the tax relief and the cap would be 7k a year (as mentioned by others).

Do note that you can also transfer your cpf. So if you want to contribute more than 7k you can choose to transfer some as well.