Where Should Your Loyalty Lie?

My husband's elderly grandmother is on death's door. She lives in another state with my husband's parents and younger adult brother. My husband would like to see his grandmother very much before she dies (a few days max).

Unfortunately, it happens that I'm scheduled to undergo a medical procedure later this week (biopsy of an organ) as I have long been suffering from a condition that requires further treatment.

My husband is of the opinion that his grandmother's condition outweighs mine and that I am being selfish for not postponing my procedures. Am I?