Rude or not!

No, I don’t think it would be rude at all. If she has a plate on her lap you could do the same and have a biscuit. Easier than going out. You give the impression that she doesn’t converse much, but see no reason why you wouldn’t tell her that you usually eat later.

If she is like the people I know like this in real life, she wont mind at all if she eats at your house, at her time of eating, without you eating.

Yes, I understand this. My MiL use to puff and sigh and look at her watch also. We eat normally around 6 - 6.30pm but on the days she came we just adjusted as it was only for a couple of times - later brunch? earlier dinner ?and as she goes home at 6.30 then a nice supper just the two of you.

On the days she's with you I'd have brunch and then eat at "tea time" (4 or 4.30) with her as she prefers.

Your brother usually does it so obviously it's possible - and it only needs to be done for another couple of weeks after all.

This is what I'd do

Then I'd run her home at six () and maybe have a simple supper later with my lovely husband - or perhaps just cheese and biscuits.

Don't forget how nice it is to eat in company. She may be "used to" eating in front of the telly and not be very chatty - but presumably she gets quite a lot out of it being at the heart of things.

When my mother lived with us she was used to having her main meal at lunch-time rather than the evening when we normally ate.
It's probably better to eat earlier, although we still don't.

The main thing is to feed her when she needs to eat, no matter if you want to eat later.

My mother-in-law was a very slow eater when she got older and was used to eat in front of the TV but when she stayed she ate with us. Many's the time the children would excuse themselves but she was still eating so I felt obliged to sit with her.

She is very deaf so conversation isn't easy. I think she likes the change of scenery (briefly) but is only happy in her own home