There are numerous causes related to canker sores. All of them seem to be due to some type of stress, whether emotional, physical, or nutritional. Sometimes even excessive exposure to sunlight can be a causative factor. Whatever the cause, canker sores, like all other physical ailments are the result of an imbalance in your body's biochemistry.
Some of the most common recommendations for canker sores involve the amino acid L-Lysine and Vitamin C complex with bioflavonoids. The dosage that I usually recommend is 500 mg. of L-Lysine with 500 mg. of Vitamin C complex with each meal and at bedtime. That will help the canker sores heal more rapidly. Some people use 1/4 to 1/2 that dosage for maintenance. For patients that I can see and test in my clinic I recommend enzyme formulas to stop canker sores even more rapidly.
Without that option, correcting dietary imbalances is the best home remedy. Your sister should emphasize using fresh, organically grown or raised foods whenever possible. Her basic diet should be lighter meats such as chicken, turkey, fish, and wild game. One of the best sources of protein is organically raised eggs. Fresh, deep ocean fish is also excellent. In addition, she should use lots of fresh vegetables with her protein, especially the dark, green, leafy vegies. Equally important is plenty of pure water daily.
There is also a list of foods that can worsen conditions that result in canker sores. The foods that can cause a flareup are lower in lysine and the foods that help slow or stop flareups are high in lysine. If you cannot find such a list on the Internet, let me know and I can get the information to you.
Best of Health,