This is my first read through, so I'm only going off what I've seen so far, but there's a lot of signs pointing to Nino being the bride. Firstly, she seems to recognise the younger version of Uesugi fom the photo, then there's him wearing the bracelet he gave to her at his wedding, and finally the panel where she enters shows a close up of her profile wearing earings, and a number of chapters ago she said something along the lines of wanting her ears to be pierced in time for her wedding. Not to mention I honestly think compared to most of the others (aside from Miku), she has had the most amount of intimate moments with him.
Personally I'm still rooting for Miku though.
Or failing that, harem ending.