What you're experiencing with them is what's known as a "de-clanning". At that age it's not unknown, especially in females, which are the dominant sex in gerbils. Basically they've just got sick of each other and if you don't separate them Angel will probably kill her sister. Even though they look ok now, it's only a matter of time.
The sister (sorry, you didn't mention her name) will be lonely so you'll have to get her a companion. The younger the new one is, the easier it will be to introduce them. You mustn't just stick them in the cage/tank together when you do introduce though, because they'll just kill each other. You need to do what's called a split tank introduction. Basically you have to use very fine wire mesh (gaps half a centimetre or less, my advice is to get an old fire screen and cut it up) to divide the tank in two, be it diagonally or down the middle as you look at the tank. One gerbil goes on each side of the divide. Be ABSOLUTELY 100% CERTAIN that they can't get over, under or around the divide! I can't stress this enough. The two gerbils need food and water on both sides, obviously, but other than that they should have only about 2 inches of woodshavings/bedding, some nesting material and maybe a cardboard tube and nothing else. This is so they don't just hide from each other or distract themselves with toys and defeat the object of the exercise. You need to take the gerbils out and swap them round to each other's sides of the tank about 4-6 times a day, or as often as you can manage with work. Once when you get up, once at lunch, once at dinner and once at bedtime would be a good schedule. You must watch them for 1-2 weeks or more, swapping like this every day. When they stop stomping their feet and lunging at each other (if they do that, plenty don't), and when they're not scent-marking all over the place with their belly gland, this is a good sign they're accepting each other's smell. A sure sign you're ready to try removing the divide is when they sniff and groom each other through the mesh, sleep in each other's nests and sometimes cuddle up to sleep next to each other at the divide.
When you do remove the divide, make sure you have the time to watch them for several hours. It's by no means certain that the intro will be a success even if the signs are all good. You may have to put the divide back in and try again a few days later. It's normal for there to be a bit of foot-stomping and skittishness but any rolling up in a ball together and attacking each other WILL end up in the death of one or both if they aren't separated. For this reason, you might want to have some heavy gardening gloves on when you remove the mesh because in the frenzy of a gerbil fight they'll bite anything that touches them and you will get bitten when you separate them so it's best to be protected.
As to your other questions, first of all YES! Gerbils love sand baths. They should have access to them at least once or twice a week, whether they look dirty or not. Use chinchilla sand or children's play sand. If when you pour the sand you get clouds of dust, this means it's too fine and you need to try another brand.
The diet issue - extra protein would be good for them, especially if their coat looks funny or their whiskers seem to have broken off. Give them cat or dog biscuits or dry cat/dog food. The stuff for puppies and kittens is good because it often has extra calcium in it. If you're brave enough you can give them crickets or mealworms but you need to supervise them while they have any live food and take out anything they leave behind otherwise it'll go rotten and make them sick. To bulk the girls up you might also give them sunflower seeds, unsalted raw peanuts/pine nuts/pumpkin seeds/just about any nuts or seeds really, raisins, banana chips (without honey or coating of any kind), dried fruit such as prunes or dried apricots etc, millet sprays (usually sold in the birdseed section) or unsweetened porridge oats. Be sure they have their usual mix too though because these are just extras I've mentioned.
Hopefully this helps
