Question

Why does this pie have less or fewer peaches than the others.

Grandmother took my brother and I or me to buy candles

Answer 1

You use "less" when you have a non-count amount (such as milk or sand), and you use "fewer" when you have a countable amount. I'd say, "Why does this pie have fewer peaches than the others do?"

"I" is used for subjects or predicate nouns; "me" is used for objects (direct objects, indirect objects, objects of prepositions ... ). Since that sentence already has a subject (Grandmother), you need to use "me" -- it's one of the direct objects.