There are several explanations. First of all, my sister, especially when she's older brother, is experiencing feelings for him is like a mother's love - in her eyes he grew up, took his first steps, they also enjoyed successes and cope with problems. Naturally, she thinks he knows it better than anyone. Sometimes the sister of the husband does not want to lose failsafe assistant, especially if her personal life does not add up and she had to raise their children alone. In those cases, when you need a man's hand (something to beat, to help carry, repair, etc.), it immediately turns for help to his brother. The appearance of his own family is perceived as a nuisance - it's sister loses control over it and no longer can call him at any time, and require assistance.
If a brother older sister that will experience different feelings for him - she is looking for comfort, protection and support. In this case, we must try to convey to the man that you are also in need of care, you have now become an important part of his life.
Sometimes in-law directs banal female jealousy - a relative of the newly formed may be younger, prettier, luckier than her. Women with an unhappy fate can blame all their failures, and they never miss a chance to hurt harder just because "all good, but I do not."
Sometimes the cause of hostility in-law may be to protect the beloved brother of the problems and suffering. If the daughter is good, and the husband is not very noticeable, many of the relatives will subconsciously expect the moment when she left him.
Great importance is how to relate to his brother's wife, his parents - each of them afford to make the entry into the family of problematic or, conversely, easy and simple. If the family does not practice competition for love and good attitude brother and son, then daughter will feel comfortable and relaxed.