Opokua Kwapong, who lives alone in New York, was on a call with her sister, Adumea Sapong, in Manchester, when Mrs Sapong noticed that her elder sibling "didn't look right".
She raised the alarm after she also noticed her sister's voice was slurred.
Ms Kwapong said she had not realised how serious the situation was.
The 58-year-old had woken from an afternoon nap when Mrs Sapong had called her on FaceTime, a video calling feature on iPhones.
Ms Kwapong said: "My sister looked at me and said she could see that my face didn't look right.