@surreality
I did make my suggestion. If I thought that she was expressing disapproval for their relationship, I would have asked politely, calmly 'Is there a problem, ma'am?'
I have actually come around to thinking that @Catsmeow actually has a better answer of talking to the couple directly and engaging them without engaging the other person.
Do you think it would have been rude to ask your great aunt 'Is there a problem, ma'am?'
I know people with tics. My sister has cerebral palsy that gives her muscle spasms. I'm not saying this is the case with everyone, but I can't imagine my sister's mental health would be worse off for being asked if there was a problem. (And I would hope if she did have a problem that wasn't related to the couple, she would speak up and say something!) Or she might have been confused at the question and said no.
But even that little gesture does something.
Let's ask this another way. As a woman, I face microaggressions daily from men. If a man is staring at my chest, do I say nothing? Do I just accept it because I am a part of this world and I am the stupid one for getting offended and upset and putting me in a place where I feel threatened or uncomfortable? I mean, he might legitimately have zoned out and just started staring at my chest without realizing it. Should I do nothing?