If she were my mom step one would be to take the MSF course. Depending upon where you live there may still be some courses being held before the snow flys.
Then I'd have her ride your brothers 250 for a while and she how she is feeling about the whole thing. Now your brother may not think that would be too cool but she can always pull the mom card on him for awhile.
Regardless of what she does, gear, gear, gear. Helmet, jacket, gloves, and some high-top boots as a minimum and give some serious though to the pants.
When someone asks what did you do this weekend I think it would be great to say "my mom and I went on a ride for awhile".
Have fun and keep us posted.