1) Under the Hindu Succession (Amendment) Act 2005, your wife had equal coparcenary rights to her father's ancestral property (land/house in Goa and flat in Mumbai) as her brother, regardless of when she was born.
2) Because your wife died intestate and left behind children (18-year-old son) and a husband, her share of the ancestral property passes directly to you and your son under Section 15(1)(a) of the Hindu Succession Act.
3) Her mother and brother cannot force her, nor her heirs (you and your son), to sign away rights via a gift deed.
4) Since your wife was a co-owner, the ancestral property cannot be legally sold without the consent of all legal heirs, which now includes you and your son. You should immediately notify the buyers or the relevant authorities in Goa/Mumbai that the title is disputed and a partition suit is being filed.
5) As her legal heirs, you and your son should apply for mutation of records in the revenue department (for the Goa land) and the society records (for the Mumbai flat) to include your names.