Thanks you all for all your kind comments. It was very much a spontaneous moment but the lighting was already set up.
A bit of background on the circumstances. I know a couple, not too well, but enough to know that they are genuinely nice people with not a lot, but they will do whatever they can to help others. I found out by accident that they were getting married between Christmas and New Year and that the only photographs they would have would be ones from the guests' mobile phones. I'm not a wedding photographer, anyone who is has my admiration, but given the circumstances I offered to take some photos for them.
As well as taking photos of the wedding I took a couple of my studio lights to the reception and set them up in a smallish space at the back of the room, taking some photos of any of the guests who wanted them. Their mother, the wife of the best man, brought these two over as the girl wanted a photo of her and her brother and just as I was ready to press the shutter she moved, put her arm around him and her face next to his and I got this image.