Yes That's right.

Only valid 6 months.

Can reapply after 3 months only.

BUT...

If u have any valid reasons like, wife pregnant or any other medical issues u can apply for extension up to 3 months after 6 months completion by submitting the relevant documents including the dated ticket.

and also...

If u have any access for a senior person in the immigration department u can take your wife sooner legally. BUT it depends on the person u have selected.