Girls got excited while riding stallions, so it's not surprising that when they saw those guys they jumped on them. Well, their chosen pose is exactly what I pointed out in the previous sentence. It has always been a mystery why so many girls love horses, actually this video partially answers this question.
Judging by the way her sister told her brother off at the beginning of the video, she must have been decently older than he was. But that care with which she helped relieve him makes me wish I had an older sister.