[schoen]

It is basically confusing an uncle with a grandfather, or an aunt with a grandmother.

These languages are indeed relatives, literally part of the same family, but we usually distinguish whether they are lineal ancestors or not. If you said "Julia is your grandmother" when Julia was actually the person's aunt, that person might get very confused about your claim!

People's aunts do often have a cultural effect on them, but we can still be more specific and distinguish them from their grandmothers.