Is it permissible for my daughter to marry her maternal cousin given the multiple instances of suckling between the two families?

The suckling that prohibits marriage is that which reaches five known sucklings. A suckling is when the child latches onto the breast and then releases it of his own volition, while milk is present therein.

The mentioned scenarios and their effect on your daughter's engagement: 1. Your elder son suckling from this boy's mother: This has no effect on your daughter. 2. Your wife suckling this boy: If the suckling reaches five feeds, he becomes your daughter's brother; otherwise, it has no effect. 3. The boy's maternal grandmother suckling your wife alongside her elder brother: This has no effect on his marriage to your daughter, as your wife becomes his maternal aunt through suckling.

Nevertheless, it is advisable to avoid this marriage as a precautionary measure.