Hi, how did you acquire litter mate brothers? Did a breeder sell them to you? If so they might (or might not) have advice for you. Most reputable breeders do not sell litter mates because of 'litter-mate syndrome" you can Google that term to learn more about it.
Are the boys outside all of the time. Do they ever come inside. Do you have (separate) training sessions with them daily? Sorry about all the questions, but this happens a lot and most people have no idea what they have gotten themselves into, until this age, when they have wild 6 month old naughty puppies. One of the major reasons Labradors this age (6 months and up) end up at the shelter or on Craigslist.
If they are outside by themselves all the time, you will have to build them a safer structure, like a kennel with concrete blocks to prevent them digging out, for their safety; and supervise their free time in the yard since they have learned, and rewarded themselves, with breaking out.