Try and stay calm
Or she will pick up your frustration. It can get to be a kind of itch scratch cycle. If she pees in her nest and you have to remove it and clean her out, it will make her pee on everything more to re-scent mark her nest and territory.
They can be funny with litter trays and toilets and will only use it if you put the litter tray in the place where they've chosen as a toilet. If she doesn't see her sand bath as a toilet, but as a sand bath she may only pee in it when she's using it. You could leave it where it is, but would suggest a slight change.
When they have a bigger house that is large enough to fit a litter tray inside, I've found they move their toilet area into their house, and use the litter tray. They seem to like having an en suite bathroom!
Also the more substrate you have in, the less you need to do big clean outs (which stresses them) and can just spot clean mainly by taking the odd handful out and replacing it so everything else stays smelling familiar to her.
So I'd start by making sure you have at least 4 inches depth of substrate, and then change her house for a shoebox house to start with and see how that goes. Cut the bottom out of a cardboard shoe box and cut a hole in one of the long sides for a door - have the door to one side rather than in the middle. Keep the lid as a lift-off roof. Then have the shoebox house on top of the substrate and put a litter tray inside in the back corner opposite the entrance (they tend to nest on the other side, furthest away from the door where it's darker). A corner litter tray fits well, with chinchilla bathing sand in. This size is ideal and an open tray works best so they can climb in easily.
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Suggest you leave her current house in and add the shoebox house somewhere else in the tank, nearby maybe. Put a bendy bridge tunnel over the door hole you cut to make a tunnel entrance so it's dark inside. Have the litter tray inside ready and put some treats in it to tempt her in. Put a big pile of torn up strips of plain white toilet paper nearby in the cage but not in the house.
She should move in but it might take 2 or 3 days. Meanwhile don't clean her out. She should take some of the nesting material and move in and build a new nest. The change and the toilet in the larger house should help break the habit. The tunnel entrance tends to tempt them in.
If she doesn't move into it within 2 or 3 days then remove her old house and put her old nest inside the new house. If her old nest is pee'd in then keep some part of it that is still dry (even if a bit whiffy) and put that in the new house and let her rebuild her nest. Or if the whole nest is pee soaked and needs removing then just remove it and let her build a new nest with the new pile of nesting material.
There are all kinds of reasons she could be peeing in her nest, but one reason might be if she hoards under her nest and you remove the hoard when cleaning out. They get very anxious about their hoards being removed and will pee on them to deter predators (and people) from stealing them. With a larger house they have more room for their nest and hoards as well as a toilet. Although she may still tend to hoard under her nest.
So best to do less regular clean outs - if she uses the litter tray in the new house then you can just spot clean and only do a substrate change maybe every 2 to 3 months. The more substrate in there the less you use. And if you do need to remove her hoard if it's pee'd on, try to leave some of it behind that's dry and then add new food in exactly the same place, to replace some of what you've removed.
She may still pee in her sand bath as well and use both as toilets.