on your husband demise your mother had 1/3rd share in flat .
1)you have stated that you sold flat in 2010 . did your mother in law leave any will . ?
2) if she did not leave any will her 1/3rd share would devolve on all her legal heirs .
3) similarly in your native place you and your son would inherit your husband share in property belonging to his mother . if you have given your share in family property whether any document has been executed by you ?
4) if your husband relatives want to file criminal complaint of cheating let them do so . as on date no legal notice has been issued . if and when complaint is filed defend case on merits .