For TAX purposes (I am going to assume that you will file a Joint tax return with your spouse, as well as your in-laws filing a Joint tax return with each other):
If your in-laws joint income is higher than your joint income, either one of you can claim her. If their joint income is less than yours, only you can claim her.
For INSURANCE purposes, it may depend on the insurance. In most cases, you can only add a person to the insurance if that person will be a dependent. In the case of employer insurance, it would also affect the insurance being pre-tax or after-tax. So from an insurance viewpoint, it may be problematic if you claim her.
