Family Visitor Visa advice required

Hi,

I applied for my in laws family visitor visa which was granted on 13/07/19 after initial refusal and in the application we mentioned that their son (my brother in law ) who is 14yrs old will be staying with his other sister in india while in laws travel.

Now we want to apply family visit visa for him too. Do you think it is a good idea to apply For his visa? Or wait once in laws have travelled and returned?

Thanks