Shaykh Ibn Uthaymeen said:
This woman has a doubt and it is that it is not permissible for her sister’s husband to marry her so long as her sister is with him. Thus she is a Mahram [prohibited] upon him a prohibition to a certain time period and not eternally. However her understanding is incorrect because those who are prohibited to a time period are not from the Mahaarim.
Majid Jawed Al-Afghanee