who knows how to delete a no name family member?

Your sister-in-law's nephew's ex-partner Private User? - it looks like someone who got invited into your tree July 15, 2009 by someone in your familytree, took a look, removed the name and never returned.

You can try to report this person's account as abandoned. You do that in the Actions menu in the upper right corner of this person's profile.

Anyhow: Since this account was last logged in July 15, 2009 - how can this one mess up your tree tree years later?

It may look ugly to see "No Name" on the Tree there, but ---
If your sister-in-law's nephew actually has an ex-partner, and if that person is (or might be) the one who claimed the Profile and removed the info, I would suggest accept that they do not want information on themselves up on Geni, or at least not in that connection, and let it be.