It doesn't sound like your idea of personal boundaries and hers line up. I only took my son in the shower with me once or twice and he was about two and got into a terrific mess. It was just easier to get both of us clean at the same time.
If the father is in the picture and this is his mom, then he should back you up on what you feel is appropriate behavior with the baby.
If you have to deal with her on your own, you might say, "Thelma, I'm not comfortable with you doing that with Jayden. I'm okay with this (whatever "THIS" is) but I'm not okay with THAT."
My aunt and uncle came to visit me three days after my C-section, but were well mannered, stayed for a couple of hours (they travelled from out of town and wanted to see the baby before they went out of the country for several months). I know you're still recuperating at three weeks, but three weeks is not unreasonable.
Keep saying, "No, I'm not comfortable with you doing that / saying that / wanting that", nicely to her. She may never get it. She may want you to lose your cool but just be nice and firm. Good luck.