… is it because love between brothers and sisters and married people is supposed to be different?
Perhaps it is. At least the fruits of that love can be much different.
Brother and sister have the same or very similar upbringing which instills similar values, habits, traditions, ways of thought, and so on. They share many of the same strengths and weaknesses of character. If they were to marry, they would bring nothing new into their family. The shortcomings of the parents would be passed on to their children.
When two unrelated persons are attracted to each other, they tend to have similar cultural, educational, and economic backgrounds. However, they usually will have been brought up with different family habits, traditions, etc., and they will not share all the same personal strengths and weaknesses. There is a chance that they can compensate for each others shortcomings, thereby working more effectively together than they could as two separate individuals. They may also learn from each other, bring out the best in each other, and perhaps become better as individuals.
This certainly seems to be the case in my marriage.