Are squirrels related to sugar gliders?

Not at all.

Sugar gliders are marsupials, and members of the possum family. They are native to Australia.

Flying squirrels are placental mammals, and members of the rodent family. There are no squirrels, flying or otherwise, in Australia.

There is a type of glider in Australia known as a squirrel glider, but it is a glider and not a squirrel.