I find it difficult working out which of these sorts of things is the best, and even when I decide I like 'smiling' over 'smile' or whatever, I usually forget what I preferred afterwards. Perhaps I should formalise a rough schema for parents and siblings on my public tag repo and add it to my help.
That said, I think 'adjective noun' is probably the more common way of usually putting this sort of stuff–'large x'/'clean x'/'open x'. The other examples of 'closed' things, like 'legs' and 'mouth', seem to be this way around. I am not sure why 'eyes closed' would be so popular the other way around, perhaps it rolls off the mental tongue better?
Then again, for these sorts of situations, where people can reasonably differ, and there is no obviously 'true' way of putting it, I don't mind if a file has both tags until I extend the client to hide one or the other. I will probably extend sibings in future to allow people to choose which way they prefer the relationship to go. This sort of thing could be easily automated for classes of sibling like 'I prefer the translated english name of a series' vs 'I prefer the original japanese name'.