Melanie Downie shared her story on social media after her brother, Simon McGough, 37, died on May 6 - three years after the death of her other brother, William, at the age of 30.
Speaking about the two men, she said addiction had “chewed them up and spat them out”.
Mrs Downie, from Kilbirnie in North Ayrshire, warned that although the substances are marketed as “safe, fun, cheap and completely legal”, they can also be “completely deadly”.
She wrote on Facebook: “These manufacturers should be prosecuted. The websites hosting these pages should be held accountable.
“Kids and young people up and down the country are dying from drugs you can buy online. How many more families have to go through this?
“Please don’t feel sorry for me. Feel sorry for my two poor brothers. Feel sorry for my mum and dad, from three children to one in the space of three years.
“Feel sorry for every single addict up and down this country with no support available, nowhere to turn to.
“Legal highs should be banned. The danger of these drugs needs to be highlighted.”
In 2014 legal highs were said to be linked to 112 deaths in Scotland.