I teach "50+" yoga that ranges from a regular hatha yoga class if the people who show up are pretty fit, to chair yoga if the folks with mobility issues - including walkers and wheelchairs - show up. When we do a chair routine, the more able-bodied always express surprise that they got a good workout as well!
"ageless yoga" is another good search term that might bring up some different dvds than "chair yoga".
If you are coaching your mother yourself, I'd suggest keeping alignment tips to a minimum, unless she enjoys it -- more important to keep her moving and keep her happy.
Remember, yoga is for life, not life for yoga. If yoga is a good way for you and your mom to connect, wonderful! otherwise, spend your time with her playing mah jong or whatever she likes to do so that you have some happy times with her.
Yoga is not 'really' so much about alignment, or even health... imho the most 'important' yoga is finding balance and creating love.
Blessings to you and your mom
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