[Opinions] I'd suggest Avery or Addison.
I know Valerie can be a boys' name, but I don't think it's nearly as commonly used for American boys as Whitney and Courtney. I wouldn't be at all surprised to meet a male Whitney or Courtney, though I assume people with those names are female because they're more popular for girls and have been for a while.
If your brother and SIL like "unisex" names for their sons, I'd suggest Avery or Addison. I think either one sounds nice with Whitney and Courtney and aren't too uncommon to see on boys even though they're quite popular for girls.