Bunnies can start having greens at 12 weeks of age, so it’s fine that yours are eating them. You do want to go slowly when first introducing greens. At their age, they are still growing and will benefit from eating pellets. If they don’t like the new ones, go back to the first brand. As to the quantity, it’s usually stated that young rabbits, under 6 months old, can have unlimited pellets. But you really don’t want them to eat too many, so don’t keep refilling the bowl if they empty it in a short amount of time. Maybe you could try 1/2 cup of pellets a day, per each rabbit, so if they eat out of the same bowl, give them 1 cup each day. If they don’t finish that in one day, you can decrease it a little. After 6 months, you will limit pellets and feed by weight. A medium size rabbit (5 to 7 pounds) would get about 1/4 cup of pellets a day. The pellets should be plain, with no seeds or colored bits added.
I assume that your rabbits aren’t neutered. Even though they are brothers and have grown up together, at 16 weeks of age they should not be together if they are not neutered. Up to now, they have had a baby bunny bond, but that stops when the rabbits hit puberty. When the hormones come flooding in, there will likely be aggression, territory issues, etc. With aggression, there is fighting. Rabbits can severely injure each other when they fight. They will need to be housed separately and cannot be let out for play and exercise together. After they have been neutered and about a month has passed, you can start the bonding procedure, to ensure that they will have a permanent bond.
Here are 2 good articles on bonding:
http://rabbit.org/faq-bonding-multiple-rabbits/
https://binkybunny.com/BUNNYINFO/tabid/53/CategoryID/9/PID/940/Default.aspx