Usually, you can add a vehicle to your spouse's policy by an endorsement. (An endorsement is a document which is attached to the policy that modifies or changes the original policy in some way.) You will need to let your wife's insurance company know your mother is also on the registration. Some insurance companies may require you to remove your mother from the registration. Other companies will allow her name to remain on the registration and will ignore her entirely. Still, other companies will allow her name to remain on the registration and give you a choice. They will tell you to either list her as an additional driver on the policy. Or, they will tell you to specifically exclude her from the policy. (If you have to list her, and you choose to exclude her, the insurance company will probably have an extra form for you to sign.)
If adding a vehicle to your wife's policy does not reflect in a multi-car discount auto insurance premium, click here to get a quote.