Let me offer a different perspective.
I agree with Myriads that substituting “fuck” for “tickle” is a good way to illustrate the concern. I don’t think it’s a perfect analogy, though, in part because intercourse generally is “you did it or you didn’t.” For tickling, on the other hand, grabbing a foot and giving it a 3 second tickle is not analogous to intercourse, though tying someone up naked and tickling them . . .
OP, you have a great situation with your gf. There is a very serious risk that her learning of your fetish will make her very upset that you’ve been tickling her mom. As I see it, you have 2 basic options.
1. Continue to hide the fetish from her. Good for the short-term, but if you’re looking for something long-term, I think she will eventually find out, and the farther out that is, the greater the negative reaction.
2. A disclosure plan, that would go something like this:
a. Keep your hands off her mom;
b. After you’ve kept your hands off her mom for a good while, tell your gf about your fetish.
c. When she expresses outrage about you tickling her mom, tell her that it wasn’t sexual for you, but at some point, you realized that it could be seen that way, so you stopped.
Good luck!