The sister's husband is not considered a Mahram to his sister-in-law, even if it is he who brought her up or supported her financially. Therefore, she is considered Ajnabiyyah (a woman other than a wife or unmarriageable female relatives) to him. Likewise, it is not permissible for him to be in Khulwah (being alone with a member of the opposite sex) with her unless his wife had breast-fed her sister five times or more during the first two years of her life. This way the husband would be her foster-father and hence a Mahram by a suckling relation. May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.