J and K are siblings. M is the maternal uncle of J. S is the maternal grandmother of K. A is the only daughter of S. How is A related to M?

Let's sketch the family links step-by-step:
J and K are siblings (sharing the same mother and father).

M is the maternal uncle of J. This means M is the brother of J's mother.

S is the maternal grandmother of K. Since J and K are siblings, S is also the maternal grandmother of J. Thus, S is the mother of J's mother.

Since M is the brother of J's mother, S is also the mother of M.

"A is the only daughter of S". Since S's daughter is the mother of J and K, and she is the only daughter of S, A must be the mother of J and K.

Since A is the mother of J, and M is the maternal uncle of J (brother of J's mother), A and M are siblings. Specifically, A is the sister of M.