Edit a profile, on the right side, it has a 'Siblings' section. Select the Add / Remove sibling and either enter a Wikitree ID at the top or enter the new profile information. Just above the 'Add Sibling' button, it has checkboxes on the same parents as the person whose profile you were on. If either of those parents is different, make sure to unclick the incorrect parent. You will have to then add that missing parent to the new profile.
As previously stated, you can also add a child from one of the Parent's profiles.