I've only dealt with the struvite crystals but I know that making sure that your cat gets plenty of water is very important. My cat Storm had to have the PU surgery because he became blocked 2 times and diet alone wasn't helping him. He was eating Royal Canin Urinary S/O which is good for both the struvite and calcium oxalate crystals. I gave him both dry and canned food and I would mix in some extra water with the canned food. Your sister could also mix some dry and canned food together with some extra water.

Now Storm is eating a grain free diet which is Innova Evo and this has been approved by both my regular vet and my holistic vet. He gets about 1/2 cup of dry a day and 2 small helpings of canned mixed with extra water each day. He's had no more UTI's or crystals in his urine since being on this food. I know that Catty1 had started a thread about grain free foods being good for cats with urinary and crystal problems here's the link: http://petoftheday.com/talk/showthread.php?t=151655.

I think that many vets will not agree with the grain free diets but many of them don't know much about nutrition so they think that only the prescription diets are the ones to use. Good luck.