Estrangement is built on disappointments and misunderstandings. Estrangement is complicated, and as deep-seated as the primary attachment that twins share at birth and throughout their childhoods. It is not that twins really hate their twin per se. More likely, twins who cannot be together are afraid of each other and the anger and disappointment that they share. My sister and I are estranged because I am afraid of her anger and disappointment with me. Other twins are just plain angry at one another and are happy to not be together at all. Anger is based of disappointment, not necessarily hatred. Estrangement always contains lost love and lost attachment.
Twin loss is very similar to estrangement but more final. There is no hope when your twin dies that you will be able to feel their closeness and warmth. Estranged twins hope that perhaps someday they will be able to be together, even if it seems so unlikely. Both estrangement and loss create a deep loneliness in twins that is hard to overcome.