Third wife

I think the marriage is valid under Muslim law.

Sir, I would like to correct my answer. He cannot marry his wife's sister. Pl go thru the following :

Forbidden to you (for marriage) are: two sisters in wedlock at the same time, except for what has already passed. [al-Nisaa 4:23]

"It was reported from Abu Hurayrah that the Messenger of Allaah (peace and blessings of Allaah be upon him) forbade marrying a woman then her paternal aunt, or a paternal aunt then her brother's daughter, a woman then her maternal aunt, or a maternal aunt then her sister's daughter, or an older sister then the younger sister, or a younger sister then the older sister." (Narrated by al-Tirmidhi, no. 1045; Abu Dawood, no. 1768. Al-Tirmidhi said, it is hasan saheeh).

according to muslim law , a muslim can make 4 wife.

yes Suchitra is correct a musilm male cannot marry his wifes sister its forebidden.

Thanks Suchitra