K.F.
It happened and it's over. You apologized now let it go. Stop talking about it, stop thinking about it and get on with enjoying your summer. There is nothing more left to be done.
I wouldn't avoid the sister, that would be childish to me. I would just let it go remind whoever brings it up that it happened awhile ago and the kids have gotten past it and so should we. I would just leave the pool toys at home as to not have the item that caused the conflict around but I would just move on. Really it shouldn't be that serious. These things happen and you have the rest of the summer to enjoy so enjoy it without the added drama.