Question

x says pointing to Y, he is my sisters only brothers son. How is Y related

Answer 1

Since it's the son of his only sister, that's the speaker. And that person's son means the speakers son. Therefore X is Y's father.

Answer 2

X points to Y and says, he is my sister’s only brother’s son.

Let Z be the sister who has a single brother.

Therefore, naturally, X will be the brother. Again Y is the nephew of Z.

Since z has only one brother and that is X, therefore Y is the son of X.

Hence X and y are related as father and the son.