As far as adding the new relationship, most genealogy software programs will allow you to add a father, mother, spouse, etc to an individual and gives you the option to select an existing person in the tree rather than add a new person.
For example Jane married John. John dies and Jane marries a second time to John's brother George. George is already in your tree, so instead of creating a duplicate Jane, you just add a marriage and select George from your list of existing people in the tree. The thing to keep in mind is that you can have multiple "relationships" to other people, but you are still one "person".
For example Jane married John. John dies and Jane marries a second time to John's brother George. George is already in your tree, so instead of creating a duplicate Jane, you just add a marriage and select George from your list of existing people in the tree. The thing to keep in mind is that you can have multiple "relationships" to other people, but you are still one "person".