Who is leta lestrange to bellatrix?

Nobody.

Leta Lestrange existed 54 years before Bellatrix. They just have the last name “Lestrange” in common. And Bellatrix wasn't even a Lestrange by birth — she was a Black and became a Lestrange after he married Rodolphus. Her husband Rodolphus was a Lestrange.

Lestrange family is one of the oldest pure-blood families and existed even long time before the Harry Potter story begins.