A baby does not need to be fed for 90 min for a feed. About 10-15 minutes at each breast is an adequate time for feeding. Most babies, when satisfied, would spontaneously leave the breast. You should continue to exclusively breast feed your child till 6 months of age.
There is nothing wrong with your feeding your younger brothers. However, make sure it is not at the expense of your own baby's feeding. He is young and should be given only breast milk while your brothers at 15 months do not really need breast feeds for their nutritional requirements. They should be taking semi-solid & solid feeds with breast milk only supplementing this. If you have excess breast milk after your baby is satiated, you can feed your brothers.