How Can I Get My Mom to Take a Shower with Me?
Maybe the best thing that you can do in order to let your mother knew that you wanted to take a shower along with her is to tell her directly. If she's available, then I see no reason that she will turn you down.
Getting your mother to shower with you is only necessary if you are a child, or unable to wash yourself. You can ask her politely if you want your mother to help you shower.