Beth,
There are several ways you and your sister can start sharing expenses now that she will be living with you. There is a referred to category of insurance titled Female Auto Insurance Phoenix which is one of them. Call your current auto insurance carrier and ask how much it will be to add her to your policy. Make sure to ask for special discounts such as an age discount since she is over 25 and a female auto insurance rate since she is a she. Make sure to check on a female auto insurance Phoenix rate for yourself along with any other discounts that may be available for you. It is also good to get as many quotes as you can. The insurance industry is very competitive, so you may be able to get a far cheaper one by just researching your options. Online is a great way to research since you can get several quotes all at the same time.