This doesn't always show the way I would prefer, but it is always accurate:
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Both sisters were already on the tree (they are biologically related to the rest of the family).
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Add the husband of each sister normally.
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Add the parents of one of the husbands/brothers as you normally would
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Add the parents of the other husband/brother by using "More actions..." then "set parents". You can choose from parents already in the tree with this option.
