Someone from Arge's past life has miraculously caught up with her. And that person is a relative too. Fortunately she does not seem to be an enemy, not on Earth and not in this world.
Interrupted confession trope. Sure, they were relatives in their past lives but not anymore. But she'll have to get in line, IIRC Arge already has a harem of admirers and now that includes both Subaru and Aoba.
What I find unbelievable is the influence of his old family bleeding over into that new world. To have two members from the same clan reincarnate there is already crazy, more is unbelievable without pulling some strings with gods in charge of reincarnation.