Just tell her what you told us. Tell her it makes you feel awkward and that you don't care what they do in the privacy of a home, but they should be more respectful of the situation.
Yeah I hate that too! You're probably going to have to tell her privately that you feel like a 5th wheel (unwanted) when she starts that stuff, like you feel she wishes you weren't around. If you tell her how it makes you feel, she should be sensitive enough to not want you to feel bad. Then you're not telling her off, you're only making her aware of your discomfort.
If she still does it, either walk off, or say loudly, "If you can kiss him so can I!" or "Excuse me did I just become invisible here?" or "Ok, I know when I'm not wanted." Best to not hang around them when they act like that so they get the message.