dnd 5e – Can you use twinned spell metamagic on a spell cast from a ring of spell storing?

Yes, you can.

You can apply Metamagic to a spell that you cast through some means other than your own spell slots. The metamagic texts only say “when you cast a spell” (or similar language), so you can apply them to spells that originate from items as long as those items say “you cast” (or again, similar language).

As an example, the Staff of the Woodlands says “You can use an action to expend 1 or more of the staff’s charges to cast one of the following spells from it”, and the Wand of Web says “you can use an action to expend 1 of its charges to cast the web spell”.

Since the Ring of Spell Storing says “While wearing this ring, you can cast any spell stored in it” and “(the spell) is otherwise treated as if you cast the spell”, you’re casting the spell, so it’s valid for metamagic.

Counter-examples would be the Necklace of Fireballs, which says “You can use an action to detach a bead and throw it (…) the bead detonates as a 3rd-level fireball spell”, or the Armor of Etherealness, which lets you “gain the effect of the etherealness spell”. Since they don’t say you cast the spell, you didn’t cast anything, and there’s nothing to metamagic.