Is it permissible to see the wife’s mother without a hijab and to shake hands with her after the marriage contract but before consummation, and is there any sin in previous interactions if it was unlawful or reprehensible?

Once a man contracts marriage with a woman, her mother becomes his mahram (unmarriageable kin) permanently. It is permissible for him to shake hands with her, be in seclusion with her, travel with her, and for her to appear before him with her hair uncovered. Imam Ahmad explicitly stated this, and it is the view of the majority of scholars, who cite Allah's saying: "And your wives' mothers." The mother of a woman engaged to be married is a ghair mahram (stranger) before the marriage contract, but after the contract, she becomes his mahram.