You can pursue her normally if you are single. Whether you can succeed depends on whether you two have fate or not. If you have a family, you should address her as elder sister. And really treat her as a respected sister. Never treat her as a lover. This is a great harm to you, your family and her. There is a good saying: don't do to others what you don't want them to do to you. You can put yourself in others' shoes. If your wife knew a single man, how would you feel if she took him as a lover? You must not do anything that hurts your conscience and your family in just a few decades.