Why not compromise and eat at 6.00, or whenever falls in between your routine and hers?
I think it's quite rude to insist, as a guest in someone's home, that everyone adapts to your own preference for eating, which is all it is, but as host, I would be prepared to shuffle things around a bit.
If someone is, say, diabetic, and needs to eat at particular times, it's different, but a preference for eating at one time shouldn't overrule the preference of others to do likewise at a different time. I think in this situation that it's your mum who is being rude.
I remember my grandmother coming to us for Christmas when I was a child, and wanting to eat at 12.00, as that was her preference. It completely disrupted the day, as we were used to eating much later, and my mum had to get up extremely early to start preparing - she missed a lot of the present opening and so on, to accommodate granny 'because she is old'.
If I went to someone else's house, and was being treated to meals, company and whatever else, I would be happy to fit around their needs and preferences - why not?
I wouldn't have two sittings, but I might ask if she'd prefer a sandwich/omelette at her preferred time, and she could sit with everyone else at the house's mealtime who the rest of the family were eating. I wouldn't expect everyone else to fit around one person without good reason.
