"You're one of the only people" plurality

Solution 1:

To determine agreement, you need to find the head noun being modified by the relative clause who is/are nice to me. First let's find the head noun candidates . . .

You're one of the only [people] [who are nice to me].

You're [one] of the only people [who is nice to me].

Now let's test them . . .

You're one of the only people who are nice to me. --> Of the only people who are nice to me, you're one.

You're one of the only people who is nice to me. --> Of the only people, you're one who is nice to me.* (incorrect—which only people?)

Sometimes you do have a choice! Here are some more head noun candidates . . .

Gruyère is one of the Swiss [cheeses] [that are used in fondue].

Gruyère is [one] of the Swiss cheeses [that is used in fondue].

Again, let's test them . . .

Gruyère is one of the Swiss cheeses that are used in fondue. --> Of the Swiss cheeses that are used in fondue, Gruyère is one.

Gruyère is one of the Swiss cheeses that is used in fondue. --> Of the Swiss cheeses, Gruyère is one that is used in fondue.

Deciding whether something (e.g. one) is a pre-modifying quantifier or a head noun can be tricky. I imagine any rule that might exist would be way more complicated than applying the tests. Meanwhile, you can always write your way out of the situation . . .

You're among the few people who are nice to me.

Further reading:

Quantity Phrase Nouns—Agreement with Verb in Relative Clause