You can not marry your sister-in-law as long as your wife is in your nikah. It is haram for a person to marry two sisters in a time; it has been prohibited in the Holy Quran. It is not allowed for you to do the Nikah even if your wife allows, while she is in your Nikah, to marry her sister unless your wife gets separation from you completely. Both of you (you yourself and your sister-in-law) should repent sincerely from this grave sin (adultery), seek forgiveness from Allah, get separated form each other immediately and have no further relation with each other.
Allah (Subhana Wa Ta’ala) knows Best
Darul Uloom Deoband