Is it permissible to marry a man whom the mother breastfed along with the elder sister, given that the mother does not remember the number of feedings?

Prohibition is not established by doubt regarding the existence or number of sucklings, because the default is their absence. So, if the questioner's mother doubts whether she breastfed her fiancé five full sucklings or fewer, then the prohibiting suckling has not been established, and marriage is permissible. However, it is preferable and more cautious to avoid it. The suckling that establishes prohibition is five full sucklings.