Before you take any specific legal action, zero in on the specific concerns you have about the guardian's treatment of your mother-in-law. You might get the best treatment for her
If you remain convinced that the guardian -- often also called a conservator -- is not fit for the duty, then you can petition the local probate court for an order to have the guardian removed or to have the guardianship modified. This would usually require that you hire a lawyer for help. In some states, however, you can file a less formal request for such an order with the local court or judge. To find out the possibilities in your locale, contact the nearest probate court.
If you are truly concerned that the guardian is abusing your mother-in-law financially or physically -- or is neglecting her in some profound way, contact the Adult Protective Services agency in your area for help. You can discuss your concerns first on the hotline and locate a local agency if need be though the website run by the National Center on Elder Abuse.
and help salve your own concerns, by having an informal conversation with the guardian -- and in any case, you should gather as many facts about the reality of the situation before you decide to go any further.