This is a touchy subject that has come up numerous times about the morality of tickling family members.
That aside it depends more on your cousins. Do they feel comfortable with you do this?
Are you attempting to do this for fun or for sexual pleasure?
They clearly are not aware of your fetish.
So it all comes down to what is right for them because unless you are just being playfully innocent and tickling them or they actually enjoy being tickled and possibly tied up.
Add to that the simple fact that you feel the need to ask this question here proves you are not sure how to feel about it.
I don't want to spout off morals and what is right though. Even though i pretty much already did.
But the usual answer these types of threads gets is. Find a nice girlfriend who will let you tickle her.
It is not as hard as you think.
I should also mention that you are openly admitting to being 18 as of this post. Meaning you joined at age 14-15. If you are 18 now it shouldn't be a problem but then i am not sure. So be wary of that.