Is it selfish to make yourself a cup of tea in someone else's house and not ask if anyone else wants something?

My mum and I are visiting my grandparents...
They have always been kind to us to the point where they would bring us tea and biscuits to have in bed for years.
Anyways as they're now in their 80s they're tired and have a lot of aches and pains so much so they don't do what they used to.
My grandparents always say we shouldn't as for what we want if its in the house...
But I've noticed that my mum will make herself a cup of tea and she will never ask if anyone else wants one.
Or she will offer to do something and find a way to get out of doing it.
Or knowing my grandparents like the house so tidy, leave the cup she's been drinking from somewhere and walk off and forget about it.

Yesterday my grandmother asked if one of us could help with dinner, I had made the tea and done breakfast so I looked to mum. And she turned away... In the end she did cook but she kept calling my grandmother out to help her find things around the kitchen.

😔