Funny, I've recently noticed on another forum multiple threads on what to do if your neighbor is bothering you. It doesn't seem to occur to people to just politely discuss it.
If you're having that much difficulty, use your sense of humor.
Make a fake ragazine with an article about how PDA is not appropriate in some cultures and could lead to stumbling. Highlight it and pull it out during the meeting.
Put your arm around her other shoulder, weave your fingers with Sister Touchtoomuch's fingers, and gently stroke her hand with your thumb. Bonus points if you first use that hand to pick your nose vigorously.
I can think of approximately 1000 more.