Removed the yuri tag, for this seems more to be Hiei giving her congratulations to her sister for getting married.
I don't see a wedding ring in Hiei's visible hand so.. Unless pixiv tags tell otherwise.

It's...oddly no longer in pixiv, but it had a description of 指輪ごしのキス, which roughly translates to "Kiss across the ring." Thank Lazarus for form recoveries in Google Translate.

From my viewpoint, I think the hand with the ring is Hiei's, then the other hand could be either shipgirl's, just because I think it would feel awkward for Kongou to pose her left hand in-between their kiss like that -- not impossible, but kinda awkward.
...although it does still sound like Hiei's congratulating her...

I don't recall seeing descriptive pixiv tag for further clarification though, but that's just in what I recall.

But wedding ring is on the left hand, right? So it's supposed to be Kongou's hand, I think.