"I'm worried about the older child, who is still a young child too, and apparently has a pretty chaotic life...there is something going on there that isn't right"

I agree. Sometimes kids will tell the truth but mix up some of the facts such as who did what to whom (maybe because it feels safer). Perhaps this never involved your daughter at all but someone has done this thing to the other girl. Six year olds who perpetrate sexually on other kids usually have been sexually abused themselves. So hopefully your daughter is right about it never happening. If it didn't as far as she can remember, then that's probably just fine too.

Now it's time to take care of you. Continue to do your best as a parent. Give yourself grace when you make a mistake, and realize that your daughter can learn more from seeing how you handle mistakes and recover than she could if you were always 'perfect.' And if you feel guilty certainly don't feel guilty about feeling guilty! Our feelings are real whether we like them or think they are justified or not. We aren't usually in control of how we feel (otherwise most of us would never be sad, scared or ashamed!). Don't ever let anyone else tell you how to feel.

Accept the feeling of guilt (it may not be justified but if it's not going away, embrace it.) We all have made mistakes and knowing where to go with that guilt is what's important. Ask forgiveness of yourself, your higher power, or anyone else you feel you need to (including your daughter -- but just be generic "... for when I don't make all the right decisions."). Then accept the forgiveness and if you still feel guilty just remind yourself of the facts of the situation, that you have sought forgiveness, that you are making an effort not to repeat any mistakes, that overall you're doing a pretty decent job based on how wonderful your daughter is. Repeat as needed....

If needed, see a counselor for yourself. This whole event may have been more traumatic to you than to your daughter--and that would be to be expected, I would think. Parenting is a tough job. Take advantage of any healthy supports you can get...

Children's resilience is amazing! Keep enjoying your fun-loving daughter! Be there emotionally for her and she'll learn what love is from someone who knows.