1. She would need to check her employment contract but generally there could be problems working in finance if you are BR. Some companies don't allow people who are BR to work in Finance but each company is different.
2. If her husband isn't declaring BR then he won't be affected. It may be worthwhile to separate the joint current account and you sister to open a Co-op Cashminder account. Most banks won't allow BRs to have a 'normal' current account but the Co-op Cashminder account is very BR friendly.