Is it permissible to speak with one's fiancée in the presence of her elder sister's husband due to the absence of unmarriageable kin (mahram) for her?

It has been previously stated that there is no harm in the fiancée sitting with the suitor, while observing her legal hijab, and in the presence of a mahram, or someone who prevents khalwa (private seclusion), such as the presence of men and women in the gathering. As for a non-mahram man, his presence does not negate khalwa according to many scholars. If the suitor needs to speak with the fiancée, he can do so by phone or similar means. If he needs to see her and sit with her, then her sister and her sister's husband may be present in the gathering, provided that she and her sister adhere to the legal hijab and that this is done out of necessity. It must be noted that the fiancée is a stranger to the suitor until the marriage contract is concluded, and that the sister's husband is not a mahram to her.