Is it permissible for a woman who has been expelled from her home to pray Tarawih behind her sister's husband without her husband's knowledge?

If a woman prays alone behind her sister's husband, this is impermissible due to the unlawful private seclusion (khalwah). However, if there is one or more men with the husband, or other women with the woman, then there is no harm in praying behind him, provided that she does not fear temptation (fitnah). If she fears temptation, then it is forbidden.