How can one deal with a sister who is not committed to the hijab and Islamic regulations, especially since she acts with leniency in front of the husband, given the need for her presence due to unfamiliarity with the language and the desire not to lose her?

If a woman does not adhere to the Islamic dress code and interacts with foreign men in a manner that violates Sharia (definition), she is disobedient. She must be advised and reminded of Allah, warned of the danger of continuing in this state. She must be alerted that her excuses, such as her father raising her in that manner, or her husband providing her with such clothing, will not benefit her before Allah. As for her visiting you, if there is an intended benefit from it, such as advising her, then let it be. Otherwise, it is better to prevent her if you fear harm from her.