The 14 yr probably sees her as her 'sister' rather than an age gap and it's all a learning experience for the both of them. I think developing girls often 'try' with their friends at least with making out practice lol, as part of growing and learning, and it looks like you all have a close loving family relationship. So, despite the age difference they are both around the age to be curious. If the 10 yr was fine with it, and she might have asked for a kissing lesson at least (?), see it as just an adolescent learning experience they were comfortable to share together, as close siblings with a good bond. It's not a good idea to scold them if you do need to address it.
