Actually I think Kashiwagi is Kiyomine's Aunt (she is the mother of his cousin whose name I forgot). Otherwise, why would they both have different family names?
Kiyome's half-sister (Ayako) and half-brother (Masaya) have the last name Kashiwagi because they have different fathers, I think. I believe the lawyer Kashiwagi divorced and decided to change her name back. (I could be wrong.) I kind of hope Kiyomine and Takara show up, but I won't get my hopes up.
Actually Reiichi and Tsukasa's (Kiyomine's cousins who have the last name Kashiwagi) mom is introduced in the series, and she's not the lawyer. So here's my proof look at the family tree http://www.mangago.zone/read-manga/komatta_toki_ni_wa_hoshi_ni_kike/mf/v10/c003/28/ . The Kiyomine's mom's name is Mikiko and they do mention he's a lawyer. Also, just for more info the Kashiwagi lawyer's name is Mikiko http://www.mangago.zone/read-manga/super_lovers/mf/v01/c002/13/
*she's a laywer
So I'd always thought Kashiwagi(the lawyer) was somehow related to Kiyomine from Komatta Toki Ni Wa Hoshi Ni Kike!, and so I as I was rereading the series, and I found family tree, and as a matter of fact Kiyomine is the lawyer's son. So HOPEFULLY we get to seem at some point in the series, that would be so awesome to see Kiyomine and Takara, maybe even getting shocked by Haru and Ren's relationship, considering those two never passed beyond a kiss.