I just did this on purpose to see how to fix it. I made my mother her mother's sister. Then I put it back the right way. Here's how:
When you make someone the sibling of someone else, it changes their parents to the parents of the one you just connected them to.
First, get the wiki-ID's of the correct parents for the person who's in the wrong family (e.g. Allison-550).
Then bring up the profile of the person who is "misplaced" and click on edit. On the right side, you'll see links to edit the mother and father. Click edit and scroll down to where it lets you remove the parent. Do this with both parents.
Now go back to the profile of the person that was "lost" and you'll see they show no parents. Click on "add mother," then "add father," using those wiki-ID's you got earlier to put the individual back with the right parents.
Voila! ...and WHEW! :-)