I spent 6 years decluttering. It wasn't easy. Baby step by baby step. I get horribly sentimentally attached to things. Of course its the memories of people and places attached to those things rather than the objects themselves in most cases.
Somethings I circled several times before I could get rid of them (by which I mean I would circle round my home each year, something I couldn't chuck out in Year 1 might take til Year 4 to be gone - decluttering 'muscles' gain in strength with use!)
(a) she has to want to do it - sounds like she's not in the throes of 'CHAOS' syndrome yet - often the big clue - (Cant Have Anyone Over Syndrome) - the point at which many of us reached total despair and started tackling our issues with earnest.
(b) its a long journey with many side steps and slipbacks
(c) there are games you can play - eg keep the total quantum of stuff in your home the same (if you buy a new thing an old thing has to go), or counting games - I did a 16 week declutter challenge at one point - every week I had to let go of 7 books, 7 'clothing or household linen' items, 140 pieces of paper and 7 'other' things. - or, if you have a big messy room you don't know where to start - every day just get rid of 5 things - even if its an old sweet wrapper or bit of tissue - don't judge its 'worthwhileness' just get rid.
(d) there are support groups out there - Fly Lady someone mentioned and spin-offs from that - I was active on something called 'PayRoll SHEs' (side-tracked home executives) during my most active 4 years of decluttering - focused more on the needs of those who worked for a living than FlyLady's core 'stay at home mum' audience. Many of us on that group were highly educated and had dreadful problems with BOOKS - from the kind of background where it was considered nigh-on sacriledge to get rid of a book.
(e) forget about Ebaying and yardsales - that will keep your stuff hanging around for infinity. Use Freecycle and charity shops (I think in the US if you are from there you can get some kind of tax rebate from your donated items) if you want stuff to go somewhere useful. Or if you are allowed, just put it out on the side of the road with a sign 'free help yourself'.
(f) take photos of sentimental stuff.
(g) read this - I always recommend it - its a PDF file "Mortuary rites for inanimate objects" - the Japanese have 'letting go' ceremonies for different objects that have given useful service but now need to be 'let go' eg needles, spectacles, transistors...
http://www.nanzan-u.ac.jp/SHUBUNKEN/...rs/pdf/585.pdf
Maybe your mum needs to devise some kind of 'letting go' ceremony all of her own.
(h) I notice her reaction at your suggestions regarding the table setting stuff - maybe she has always had a dream of herself presiding over big family occasions - maybe you could explore that with her in more detail.
Hope something here helps you or someone!