Child adoption

I got divorced with mutual consent with my first wife. I wish to adopt the son of my second wife that is from her first marriage. My second wife is a divorcee. She got divorced with mutual consent.

Yes you can adopt the 5years old child under the Hindu Adoptions and Maintenance Act,1956 provided the natural biological parents are ready to give the child in adoption. The Adoption Deed duly signed & witnessed by both parties and registered with the Registrar of your district will be required.

I just want to be sure that I can adopt the son of the first marriage of my second wife even though I have a son from my first wife... 

Guardian & Wards Act, 1890 is for getting appointed as guardian of the minor through court against the natural guardian which in this case is the mother/second wife and the biological father, the ex-husband of the second wife. So why the natural or biological father agree for the handing over the guardianship to the new father when he is alive, rather he will ask for the same against the natural or biological mother who may otherwise agree for her new husband to be made guardian of the minor child. Even otherwise the issue of guardianship only get resolved that doesn't make the minor child adopted son of the adopting father.

As far your own son from your first wife, look the section 11(i) of HAMA may have imposed condition that there should not be son living of the adoptive father, but all this comes to play when real father challenges the adoption process in the court, till that time you may go ahead and workout the adoption deed.

In my considered opinion the child would be deprived of his status as the legal heir of his biologocal father, if the adoption, as suggested, is feasible and possible.