Litter mate syndrome

Does anyone else have 2 pups from the same litter and avoided litter mate syndrome?

My girls are 6 months old now and are best friends. They play all the time, sometimes it gets a bit rough but it's all good natured and no aggression.

Obviously I want to prevent them developing litter mate syndrome but is it guaranteed to happen with 2 from same litter?

I also have another dog but he's much older, not sure if that matters or not.

Any tips or advice gratefully received. Thanks 👍🏻