The actress shared a video of her sister, Kelly, gasping for breath earlier this week as she lies in a hospital bed. Kelly fights back tears as she begs people to take this horrible virus seriously, wear a mask, stand with testing ... and take every precaution to prevent the spread and protect others.

Kelly makes it clear -- COVID-19 is not something to mess with, and says her heart is breaking for people going through what she is.

As we reported ... Sharon's family has been ravaged during the pandemic -- she's already lost her grandmother and godmother to the coronavirus -- and her sister is fighting for life, along with her sister's husband, Bruce, who is also battling COVID.

Sharon's angry at President Trump, Montana's governor and state health officials, and everyone else who is not taking COVID seriously or acting responsibly ... despite the pain and anguish it's causing every day.

The good news ... her sister has since posted she's feeling a bit better ... but she's still under hospital care and doctors have been monitoring her lungs, which are showing signs of damage from the virus.