Being able to sex them really depends on you. Some people can sex when the kits are a few days old and others can't until they are a couple months old. The parts are very similar, so it is easy to get mixed up and even the best sometimes make mistakes.
There are some good resources and a few have already been posted. Take a look at those, then compare them to the babies. It can be helpful to have references right there when you are looking. It is a good idea to sex them again before you take any just to be sure. A second opinion doesn't hurt either.
Are you planing on keeping them together? There is no guarantee that 2 siblings will get along. Both should be spayed or neutered to help them stay together. Usually, it is a neutered male and spayed female that get along best, but if you are wanting 2 of the same sex, then females are usually better.
Has your grandmother separated the parents? If they were not separated until after the litter was discovered, she could be pregnant again. Rabbits are pregnant for about a month, so she could have another litter soon. This does mean that the current litter would need to be weaned, but should still be kept together until they are about 8 weeks to make sure they are developing well.
