Is it me misremembering or we finally got A HECKING KISS AFTER 102 CHAPTERS?
There have been a few others, but this is more undeniably romantic. First 'kiss' was back in high school, Kasumi exchanging candies/cough drops with S. Then, in or after college, Kasumi when she came home spontaneously giving S a kiss, though the art didn't make it clear whether on-the-lips or cheek-peck; it was pretty surprising to Sakurako.
My own take on their relationship, subject to the current vagaries of my memory, is to note that in senior year of high school, S was expecting them to stop living together after graduation, so she seems to have been thinking of it as "roommates with benefits", not something committed (even if she wanted that.) Kasumi proposed they keep living together. A bit after that, we get Kasumi spending all day in her panties and with her head in S's lap, jealously pushing away any interruptions, to "recharge". Then a few years later we have S calling Kasumi "you're something like my girlfriend" and K saying "I could just be your pet cat". https://dynasty-scans.com/chapters/a_room_for_two_ch53#5
So, without showing us any conventional confession, they seem to have slid from "sharing a bed as roommates" to something like a committed relationship. Whether there's any sexual activity other than very rare kisses, who knows. But if there isn't, by now it's more likely to be because Kasumi is just that asexual, rather than because "they're just friends".
Yeah, I meant it as more of an "undeniable proof that they are in love with each other" cause, as you said, there have been other moments but with angles and such they weren't as straightforward as this one, especially if you consider all the other pairings and how some even had sex behind the scenes with us knowing full well from some dialogues.