A cursory search for 上杉謙信 性別 (Uesugi Kenshin’s gender) brings up a number of sites and blog posts that discuss various arguments for Uesugi Kenshin being female. Are there any reliable sources (Japanese or otherwise) that support these claims?
Nope. As touched upon quickly here all "evidence" are incredibly circumstantial (and picked out by a non-professional, a existence of a couple of which have never been confirmed). And various aspects about his life, like getting into a succession crisis with his brother, don't make sense if he was a she.