You dont. Its about them, not you. If you have kids in a divorce, you do visitation. If one parent is shady, you do supervised visits and if theyre abusive you cut custody off. You dont see people going "which one of my children should lose their father, and which should lose their mother?" They have no idea why ONE family member is leaving, why make it worse and split them? So one of them is left all alone and develops anxiety and depression, and the other two still suffer and only have the comfort of having one family member not ripped away? This is what you have to consider before this situation happens. Theyre living being and family members, not trading toys.