I don't really have any advice to offer, other than this is the very situation I would be able to protect myself in because of my handgun permit and training. She has children and will need a gun safe, so that makes the handgun route something she needs to put thought into, but I think it's well worth the protection it offers.
I'm so sorry your sister is having to deal with this.
she's lucky to have your support esp. during times like this.
I'm glad she'll be out of that living situation soon.
you hang in there
