Oh it's hard yards!
Just keep your Mum calm and let her know she's loved. It's the little things that count like spoiling her with her favourite food or some flowers or....any little thing that makes her smile.
As to a holiday that's a wonderful idea. My Dad was unwell and my sisters convinced him to go on the holiday. His view was he might die whilst he's away and the girls said that's alright we'll deal with it if it happens as long as you enjoy the holiday. He had a blast and passed away a few weeks after the holiday. It is what it is!
As you say the Onc says she's stable so hopefully her treatment gives her many many years to come and the opportunity for you all to make happy memories.
Best wishes
Take care