I had the same problem.
When single line sibling nodes are added, they are spaced nicely.
When a number of sibling multiline nodes are added, again they are spaced nicely.
When you try to mix sibling single and multiline nodes together, they overlap and you get an unreadable mess.
Unfortunately my maps require a mix of sibling single and multiline nodes - so Freemind doesn't seem to be able to do the job.
Shame, because it is an otherwise impressive piece of software.