How do I tell grandmother she must take a shower after 3 months of not washing regularly?

My 100 yr-old aunt takes care of her 97 yr-old sister with dementia and she won't go into the shower.I think it's a dementia thing. My aunt hired a neighbor (who has experience caring for seniors) and she comes in once a week to get her to shower for a very inexpensive rate (it only takes 1 hr or so). Would your Gramma do a sponge bath? Like other commenter asked, need more info.

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Does Grandma live with you or your family, or is she living independently?

Does she have any health concerns?

Is she a fall risk?

What, if anything, has the family tried so far?

Oh Boy, how about helping to do the shower? Kind of a buddy system.