"It's important to know that she saved a little boy's life that's going to grow up and live a full life, and she is a hero," said Janice Potocki, Wessel's sister. "People need to know that there are heroes."
Wessel, described by family friend Don Bussey as a dedicated single mother, was vacationing on the lake near the far northern Wisconsin town of Plum Lake with her son, sister, a friend and two other children on Tuesday.
PHOTOS: Karen Wessel with family and friends
Emergency officials said two boys and a girl were swimming back from a sand bar across a small channel when they became very tired and began to struggle.
Wessel, 47, and two other women jumped into the water to save them. Two made it back safely. She drowned while saving the last child.
Rescue crews were able to pull Wessel out of the lake, but she died at the hospital.
The Daily Herald reports Wessel was familiar with Star Lake and her family had a vacation home near the water.
Janice Potocki and Karen Wessel lived next door to one another. Potocki said her sister spent her life giving.
"It was everything about every other person in the universe except herself," Potocki said. "I just keep expecting that to happen... I keep expecting that when I open my refrigerator there will be dinner - because she would just leave me a little plate in case I was hungry."
Wessel was a secretary at a Chicago law firm. She's survived by her 9-year-old son Michael and 17-year-old daughter, Elizabeth.
A friend of the family has set up a scholarship fund at the Village Bank of Arlington Heights to help support him and the family.