We are simply asking a Customer who must lift the armrest in order to sit in the aircraft seat and who, at that point, is obviously occupying a portion (or all) of the seat next to him/her, to pay for the additional seat being occupied. Again, we will offer a refund if the flight does not oversell.
Let's assume for a moment that you were not traveling with your brother. Could you physically fit in between the two armrests (regardless of whether or not you were comfortable?) If the answer is yes, you won't have any problems. If the answer is no, you would be asked to purchase a second seat. That's the way I am interpreting this rule.