I'm very sorry about your sister - I also know someone who had a stroke, early last year, so I know what that's like. And she is convinced it was linked to the vaccine she had shortly before, and has been reluctant to have boosters since.
But in reality, both of our lived experience is no more than that we know someone who had a stroke, who also had a covid vaccine shortly beforehand. The reality is that most people who had strokes in 2021/22 will have had a vaccine dose shortly before, because nearly everyone was having the vaccine.
The stroke cases associated with the AZ vaccine were very rare, so the chance of it being associated is very tiny. And the risk of having a stroke as a result of contracting Covid is much much higher.