Is it permissible for me to marry my wife’s sister forty-eight days of my wife’s death?

Yes, it is permissible. What is prohibited is marrying two sisters simultaneously, as Allah Almighty says regarding the women prohibited in marriage, “Prohibited to you [for marriage] … that you take [in marriage] two sisters simultaneously” (Quran, 4: 23). Therefore, if a wife passes away, it is permissible for her widower to marry her sister without having to wait for any period of time. Similarly, following an irrevocable divorce, the majority of scholars maintain that it is permissible for the former husband to marry his former wife’s sister without waiting for any period to elapse. However, in Hanafi jurisprudence, the husband must wait until his divorcee’s waiting period has expired.

And Allah Almighty knows best.