In the rare instances that Shareen Williams can treat her younger sister to something special, like a nail salon pampering for her birthday, it comes with a twinge of regret.

“Sometimes I feel so guilty when I feel like I’m not able to give her just little stuff that I feel she deserves,” said Ms. Williams, 35, who has cared for Raheida Harry, 21, for more than a decade. In May, Ms. Williams became Ms. Harry’s legal guardian.

“I put myself on the back burner a lot, just for her,” Ms. Williams said.

Ms. Harry has had developmental delays since birth, but the underlying condition went undiagnosed for years. The sisters were raised on St. Vincent, the largest island in the Caribbean nation St. Vincent and the Grenadines. It is a place, Ms. Williams said, where her sister was unable to thrive, let alone receive adequate services, because of the limited resources for people with special needs.

Twelve years ago, their mother left St. Vincent to live with a relative in the United States. Ms. Williams had to step in as her sister’s caretaker and provider.