Yes, you have to be present in order to receive the cruise discount. But last I checked, you do not need to be employed for 6 months in order to receive the discount. I had only been working for 3 months and was already able to take advantage of the cruise discount, provided the ship was not booked to capacity with regular paying guests.
You, as the cast member, have to be in the cabin with your brother. The discount only counts if you are there. By the way, you have to be working there for a minimum of 6 months before you can get the cruise discount.
I believe you have to go with him because Disney is cutting back on how much discount they give to their employee s family now. You know these are tough times for everyone including Disney