Other parents with experience with sibling experimentation?

I am a very open minded person so my approach would probably be different than yours but I would say to have a conversation with the 14 year old but don't overreact. She may just be trying to figure herself out at that age.

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.

My grandma caught me and female cousin doing more than kissing when we were very little. She snapped the cover off and yanked me out of the bed. There was a lot of shaming involved. DO NOT SHAME THEM!!! Be understanding. We were all curious at one point.

When you say she was "teaching," what exactly do you mean?
Was she teaching her by explaining it to her or was she actually teaching it by doing it with her?
With explaining, I don't see a problem but IMO, actually being physical may cause problems for both girls in the future.

It's just learning experience. I say good for them and props to your older daughter for being a great sister and teacher to her little sister

It’s all normal experimentation and at that age innocent.

I wouldn’t make a big thing of it as it’s just what we did at that age.

sounds very normal to me. have you forgotten your own curiosity and experimenting.

What does it mean, a little more?

As I got caught kissing the neighbors girl when I was 12 and she 9, I got 10 slaps on each face cheek and my mouth washed out with soap.
I never did it again.

I wouldn’t overreact I would have a conversation with her and maybe grounding her for a weekend

What else

Let’s chat…send me a DM.

What are you worried about?