From the beginning, take a hands-off-but-watchful approach to their relationship. Kids bicker, squabble, fight, whine, and drive each other bonkers—it's part of what kids do. Of course watch for danger, but let them take responsibility for their relationship as much as possible. The sooner they learn to solve their own struggles and disagreements, the better off you'll all be.
Remember that the family is a wonderful place for kids to learn conflict resolution skills. Only children have a harder row to hoe. (Ever see two only children playing with each other? They often act like siblings!)
When things get rough and there's no easy resolution in sight, you can help by translating for each other. If things are really rough, they may need a cool down period. In other words, you may need to separate them. But try not to just leave it there; bring them back together to work it out later on.
Trust in their abilities. Focus on their strengths, not weaknesses or disabilities.
Make time as a family to do things together. Shared experiences are the building blocks of strong relationships and help build solid family loyalty.
Kids also need time—and activities—away from each other. Help each find an area (or areas) in which he or she can flower.
Never compare your kids, type your kids, or allow others to label them. You can't know the full extent of their abilities and talents unless you let them grow.
Sibling love is one of the strongest bonds on the planet—allow the loyalty to grow. Seeing your kids working together, teaching each other, loving, playing (and even conspiring against their parents) will make it all worthwhile—all the "Mommy, Jake is bugging me," all the practical jokes, all the tears.