DB, there is no problem at all. The "du" has no expiry date, and, as stated above, taking it back would need something like you stole her heavy silver gravy ladle and she can prove it. Then she might clearly pronounce her unwillingness to use "du."

Some of the posters seem to assume your ffmil reverted back to "Sie," which very obviously is not the case. So you needn't even ask you ffbil. "Du" it is. Period.