Who can perform barsi — only the eldest son?

The eldest son is the default performer under Dharmashastra, but the Shastric hierarchy provides clear alternatives: eldest son, then any other son (by birth order), then grandson through the male line, then Dauhitra (grandson through a daughter — explicitly validated in Manu Smriti and Vishwamitra Smriti), then any male Sapinda relative within seven generations, and finally the wife of the deceased if no male relative is available. The fundamental requirement is sincere Shraddha (devotion) and the willingness to perform the ceremony correctly with a qualified pandit. A family without a son should never leave the Barsi unperformed.