Oh nice, Tojiko x Mokou. The two of them - or rather their families - have some historical connections. From this Danbooru post
Tojiko's family was very powerful, and Mokou's grandfather led a coup against them, including conspiring in the assassination of Tojiko's brother's son. A day after the murder, Tojiko's brother committed suicide.
Oh, and apparently - though this part I see little proof of outside the Touhou wiki - Mokou's grandfather was later killed... by a lightning strike.