Please don't.....
Yes, she appeared very gaunt and unhealthy in recent pictures, but that could be for health reasons she kept to herself. Remember, Chadwick Boseman also didn't look to healthy during his last months but people didn't immediately jump to conclusion.
So let's NOT jump to conclusions and start nasty gossip.
As for her passing..... I remember being shocked when suddenly Buffy had a sister and wasn't sure where it would all go. In the end, Dawn was a great character and Michelle brought her to life. She will be missed.