Is it permissible for my niece to marry a religious girl accepted by her maternal cousin's son, who was nursed with her older sister, and must she cease this if the husband's family asks her to?

If the girl did not share suckling from the same woman with her maternal cousin, then he is a stranger to her (non-mahram), and it is forbidden for her to be alone with him, or to kiss him, or to touch him. Engaging in such acts is a grave sin. However, if they both suckled from the same woman five full sucklings, then he is her brother through suckling (mahram), and it is permissible for him to kiss her head or her hand when there is no fear of temptation, but kissing the mouth is forbidden. She must cease this kissing if her husband orders her to do so, because obedience to him is obligatory in matters that do not involve disobedience to Allah.