Have you tried feeding carrot tops, fresh parsley, whole grain breads, broccoli??? Some of these foods birds take to first, even lettuce, which is a start to a better diet. You must be persistent and keep feeding it to your birds in various ways so that they learn to eat it. You can try hanging it up like a toy, weaving it through the bars, dicing it up into small pieces, eating it yourself and making it a big deal about it, etc!
You might even try sprouting your own seeds. You will need seeds that do not have any dried foods inside, no pellets, no dyes (especially the dyes). You can even buy some seed from a health food store. Try checking out
www.sproutpeople.com for more information on sprouts!
As far as the seed, please read this site...
http://feistyhome.phpwebhosting.com/diet.htm especially on the letters in bold that says "My Seed is Better Than Your Seed...". There are of course some seeds that you don't really want to feed, such as feeding your budgies wild bird food (huge no no, since it contains so many fatty seeds meant for wild birds), and foods with dye (another big no no, as this can change the color of the birds droppings, and you wouldn't be sure if there is a problem or not). This is a good reason to stay away from dyed foods.
As far as the water, that is great! Bottled water, or purified water tends to be better in many places as to tap water.
As far as the no contact, well maybe not between the birds themselves, however have you ever thought about airborne diseases? Or even something that can transfer via being touched? Such as you touching Ava's cage, and later in the day, touching your sisters birds, or vice versa? Even something in the room that is around the cage could be transferred to another area of the home. So, there is still a chance that if it was a disease or virus, the other birds could get it, too.
The cage itself though sounds great! As far as collage, you'll have to consider if you'll still be able to give Ava attention, even if it's talking or singing or even reading, just to make sure that Ava stays happy. As far as the vet, he may be right, however if Ava does have something, it's better to be safe than sorry to know if Ava may be sick or not, and if Ava is sick, she could be hiding it. In the wild, birds must hide illnesses or else they are easy prey to predators. So usually when a bird is extremely sick, it can sometimes be too late to take action, though it's always worth a shot to try.