In my opinion, it's totally inappropriate for your Auntie to kiss your husband.
Him kissing her back, not the most appropriate thing to do, and he probably didn't need to tell you anything about this, but did it so HE wouldn't feel so guilty about it.

The best thing to do, just so it's acknowledged, but not made a big deal about since you do need to see her again, is to make a joke about it, like "I know my husband is hot, but you are not allowed to touch him! Don't do that again!" and then leave it alone. She should apologize but hopefully it will be quick and then done.

You guys sharing should not come into play at all - that's your choice and at your discretion.

Of course these are all my opinion but I hope it helps a little.