my sister was born on 29-5-1968 and her husband on the 17-2-1959. Are they compatible?

They both are ok. They are always polite to each other. As a result, their relationship is stable. However, they don't communicate with each other very well. For this reason, their relationship is more like a friend-to-friend realtionship. Since they have been married together, they should go further communications and make efforts to build a happy family together.