They are toxic to cats. Susie loved to drink the water they were in and lick the leaves. I told my mother I wasn't sure she should be doing that and to ask the vet about it. I trusted her to ask the vet, and Susie did, too. I guess she never did because she never stopped buying the lilies and indulging Susie. Today the poor sweet little kitty was put to sleep due to liver failure today.
Susie was nine years old, one of the sweetest cats in the world. My heart is breaking.
Always, always, always be very careful with plants and flowers around cats. If you don't know, ask! Don't assume everything is all right. If you have an elderly parent, you need to double check what they say and do, even if s/he thinks you're being obnoxious. Your parent may be not wanting to listen to you and not be as sharp as they used to be.