Section 7(3) of the DP act
The legal point that Section 7(3) attracts in this case is technically flawed because Section 7(3) applies to that particular instance of D.P. Act invoked.
This means that, had the husband used the same FIR (lodged against him) to implicate his in-laws, in that case the "complainant" (wife in this case) gets the immunity.
However, when a fresh case has been lodged, the complainant is the husband here and he is the beneficiary of the immunity provided under Section 7(3).