Good for your sister.
FIRST she needs to harden the house, and then set up a good home defense plan that will put her between the threat and her kids, and her kids in a safe place.
THEN she can figure out what gun she wants to do it with. She needs to get what suits HER best, not what a bunch of strangers on the Internet think she should get. Point her to
http://www.corneredcat.com/ for a good starting place.
http://www.ou.edu/oupd/hardhome.htm - one pamphlet on home hardening
http://www.woodarddefensivesolutions.com/node/14 (the PPITH class is taught by many NRA instructors, this is just an online course outline)