The closeness of step-siblings ... when is it "too close"?

[MamaMeercat]

My two boys (10 and 8) have known my partners daughter (12)for around 6 months and have spent alot of time together.

However my eldest DS and his daughter have become really close and whilst it's nice that they're always together, I'm wondering if they're two young for me to be "concerned" about it?

My DS is very 'beyond his years' and acts a lot older than he is. Therefore I'm worrying that they're maybe becomming "too" close.

For instance, they have started locking themselves in her bedroom. The other night they were up there for hours making no noise or anything. DS2 eventually came down complaining that he had no one to play with as "DS1 was too busy snogging his girlfriend".

DP went up to check on them and found them lying side by side on the bed writing a story together. All innocent but DP told her to leave her door open in future. We then wondered if we were being ridiculous as they were only 10 and 12. If they were 13+ I would be very worried by now.

Another thing, in the car on the way home the other day, she started to feel a little ill ... I turned around to see her cuddled up to DS. I thought "aww sweet" but in the back of my mind I thought it was a little unusual. I then heard DS whisper to her "I thought you didn't want anyone to get the wrong idea ... actually, I shouldn't have said that" and she smacked him.

Am I over-reacting or should I keep an eye on things, despite their age?

(posted this in AIBU too but think it may fit here better)

[bamboostalks]

This sounds very bizarre to me and definitely something to be concerned about. It will probably blow over but does need careful monitoring. Children can have very intense crushes when they are just on the cusp of developing their sexuality.

[BonsoirAnna]

It sounds as if they have fallen in love. There is nothing wrong with that - they are not blood relatives!

[ObsidianBlackbirdMcNight]

They aren't really step siblings though, if they have only known each other 6 months. I'm not saying you should turn a blind eye to anything (it still wouldn't be appropriate) but I think your kids would have a hard time understanding that. They don't see each other as brother and sister and they are so young, there's not a lot they can really be getting up to.
It will probably blow over (if indeed there is anything there at all)