In Family Tree Builder, the first spouse you add is considered the “primary spouse” and is displayed on the family tree by default.
In order to change the priority (primary, secondary) of a spouse, you can use the “Manage Spouses” command. In Family Tree Builder go to the genealogy card of the person who is more closely related to you among the two spouses (e.g. if your brother divorced your sister-in-law, go to your brother’s genealogy card first).
Click on the “Edit” button and choose “Spouses” then “Manage Spouses”. A popup will appear detailing all the marriages listed for that person.
In the lower right hand corner of the screen, you will see a little button that looks like a clock with a green arrow on it. Use this button to change the order of spouses and set a “primary spouse” (the one that will be shown in Tree View and also in your Immersive Tree after publishing to the Web).