Couldn’t little sibling also be Aria?
Or maybe - I have seen no mention of Brianne. I think she
could possible take Cersei out for Jaime (she likes him).
Based on the look Jaime gave Cersei he objects to what she has done...
Well, if the “little sibling” from the prophecy could be anyone’s sibling then that basically means everyone except on the planet except firstborn children are possible suspects. If that’s the case, then there wouldn’t be any point to even mentioning that in the prophecy because it wouldn’t be an identifying characteristic.
So I think it must mean either Cersei’s “little sibling”, or the “little sibling” of the younger queen who will come to supplant Cersei, since they are the only other figures mentioned in the prophecy that could possibly fit. We don’t know who the younger queen is yet, though. There was speculation it would be Margery Tyrell but she’s dead, so it can’t be her. The strongest candidates now seem to be Daenerys or Sansa Stark. If it turns out to be Daenerys, she doesn’t have any “little siblings”, but if it turns out to be Sansa, then yes, Arya could be a candidate for the “little sibling”. Jon Snow even used to call her “little sister” as a nickname, so that could be foreshadowing.