For a woman or man, all the karmas after their demise must be done sons born to them. If they have no son, the daughter’s son, especially her first son, can perform karma.
You have mentioned that for your husband, the karma is performed by your brother-in-law’s son. He would not have performed directly as the principal karta; instead, he would have performed by accepting kaippul from you. Continuing to do so will be appropriate.