What would you charge?

If it were me, I would not accept the 24-hour in-residence requirement, especially since she leaves them along all day normally. And she doesn't want you to leave the house at all? What about groceries since you'll be there over a week? That's just not realistic by any stretch of the imaginaton.

I would go with your suggestion of not leaving them alone more than 4-6 hours, and don't tell her why, it's irrelevant.

Honestly? I think that is highly infeasible. Since you also offer drop-in visits, you wouldn't be able to accept any drop-in reservations for the days leading up to and including Christmas and Near Year's Eve! For your first holiday season as a sitter, if you're wanting to just take it slow you could accept her request and charge her your holiday overnight rate plus at LEAST $10 for each additional hour, but probably more like $15-$20 (especially on the actual holidays) since it's such as busy time and you would have to miss out on seeing/being with your own family for those days. If it were me, I would respectfully advise her that I cannot offer 24hr care. I mean, I can see why she is asking for that considering her dog is on seizure medication, but even at most kennels there is not going to be someone there at ALL times, especially on holidays. Maybe offer that you wouldn't leave them for any longer than 4 or 5 hours at at time, but say that you will also be doing visits with other clients during those days. If that doesn't work for her, I would recommend that she seek someone who can board in their home who is home all day.

If she is looking for someone to act as both a house sitter and a dog sitter, then I can understand why she would want you there overnight rather than simply make drop in visits. However, if you choose to accept, I would make it clear that you will have to leave the house on rare occasions, unless she wants you to host a Christmas party while you're staying there lol