I've done some thinking and some googling and there doesn't appear to be much that is really suitable for a 90+ wheelchair user - in, possibly, iffy weather.
I did think of Wyevale Garden Centre where you could get also some lunch. I also looked at Notcutts (Woodford Garden Centre) and checked their restaurant prices out for you. Looks like you would need a second mortgage to eat there!!
I then looked at Stockport Museums
http://www.stockport.gov.uk/services/leisureculture/museumsandgalleries/?view=StandardAt a quick glance on the website it looks like The Air Raid Shelters are wheelchair friendly.
Several years ago I visited the Air Raid Shelters and it was very interesting - I do know they've opened up more tunnels since then. I've always intended revisiting but never got round to it. They don't have a cafe there but some of the other Stockport Museums do.
There is always Lyme Hall but I don't know how wheelchair friendly it is or, maybe, just a scenic drive over to Buxton or Chapel-en-le-Frith with lunch in one of the many pubs or cafes. Stocks Cafe on the market Place in Chapel is very good.
Anyway, I hope you find somewhere to take your mother-in-law.
Lily