"I am sitting between the boys."
Translation:Ik zit tussen de jongens in.
You don't really need it.
It's part of the preposition, tussen... in: together they form a unit of meaning. You can think of them as in between.
I suppose in is placed at the end of the clause because it has a similar behaviour to that of the particle in separable verbs. Just guessing, by the way, since I'm a learner just like you.