One of the most difficult challenges adult children of Alzheimer's patients face is dealing with their loved one's sexually inappropriate behavior. Many adult children, especially daughters, worry that their parent has had sexual feelings for them all along. They begin to question their whole relationship with their parent. It may come as a relief to know that acting out sexually does not make your loved one with Alzheimer's a pervert or a pedophile. The most likely explanation for your father's behavior is that he is simply mistaking you for your mother.
People with Alzheimer's tend not to realize they have aged. Ask a 75 year-old man with dementia how old he is, and he's likely to tell you he's 32 and has three small children at home. Your dad doesn't recognize you as his child--he believes his child is still a little girl. But you probably look at least somewhat like your mother did at your age. In other words, when your dad sees you, he sees your mother, and it's perfectly appropriate to kiss a spouse on the lips in greeting.One way to put a stop to this disturbing behavior is to "introduce" yourself to your father before you get within touching range. "Hi, Dad, it's your daughter, Emily. Give me a hug."
Another reason people with Alzheimer's sometimes exhibit sexually inappropriate behavior is that they are suffering from "skin hunger," or the need to be touched. People don't stop being sexual creatures or stop needing human contact just because they have aged. When your dad tries to kiss you, your first inclination is probably to pull away from him and conduct the rest of your visit from across the room.
This may actually be making the problem worse. Instead, decide what kinds of touch you are comfortable accepting from your father and what kinds you are not? It's understandable that you don't want to kiss him romantically, but how would you feel about a kiss on the cheek, a long hug, or just holding hands? The next time your father moves in for a kiss, redirect him to a more acceptable way of touch.
If your dad has trouble understanding spoken language--many people with advanced Alzheimer's do--you can use your own body language to communicate with him; for instance, turn away from the kiss, but give him a big hug. If he is still verbal, you can use words to set your boundaries as well, "Daddy, I'm your daughter, Emily. I don't want to kiss your lips, but I would like to hold your hand and go for a walk outside."
Above all, try not to be put off by your father's behavior and try not to take it personally. He is suffering from a disease that causes the tissue in the brain to shrink. As that happens, many important memories and social niceties are lost. He is relating to you the best way he knows how. Try to find a way to relate to him that you can both live with.