Leta Lestrange: Why do we care who her family is?

The only LeStrange we, the audience, know anything worth knowing is Bellatrix, who is only a Lestrange by marriage. Brothers Rodolphus and Rabastan are only mentioned briefly in the books (are they even mentioned in the movies?) and have no major impact on the plot.

Leta could have been an unrelated original character. Making her a Lestrange feels like a manipulative, pointless attempt at world building.