The relationship between Harry and Catherine have always been close since William and Catherine started dating. They have formed a good little brother and big sister type of a relationship. It’s all due to the many years they’ve known each other.
Harry was very excited to officially welcome Catherine into the family.
that does not seem to gibe with his sayng that he was looking forward to getting to know her, when W and Kate got engaged. and they are only about 2 years apart, Harldy "big sister and little brother".