Op I'm so so sorry about your mum, wishing her a speedy and full recovery and you peace of mind as soon as possible
I've nursed a lot of ladies with breast cancer.
Cotton tends to be better than satin as it washes better if stuff gets spilled on it or small blood stains but you're right on button up, they can have trouble raising their arms without pain especially if there's any lymph node surgery/treatment.
A long warm cardigan with 3/4 length loose sleeves (or 2 or 3 actually) is something she'll find very handy to warm her up but still allow easy access for IV
If she normally wears tops and doesn't have many shirts/blouses then maybe a few of those
Lip balm/moisturiser - yes but unscented no fancy scents or colourings as these can irritate
Hand cream - yes especially at the moment! I've found neutrogena Norwegian formula the best (use it myself - I have ocd and wash my hands a LOT)
A kindle is a great idea! Very easy for her to pop in her bag and means she can access not only books but also magazines which can be easier to read if concentration isn't the best or she's tired - a LONG charger lead so she can easily plug it in if the battery gets low.
Scarves - everyone thinks head warming with hair loss but if she's had longish hair till now she may well feel the chill on the back of her neck! They're a bit more versatile than hats too though of course you can do hats too. One of my patients deliberately sought out the most colourful, and aimed to have one for every potential outfit and occasion.
Good headphones - for tuning out the ward!
The things I didn’t need were endless sympathy and an aura of tragedy.
Yes I think that's generally true for most patients, if something in your life (or dcs or whoever) happens that's funny or entertaining "save it" for when you're talking to her.
A cheery upbeat approach is usually most helpful.