and Joe get married, Alexis and Brandon would be stepbrother and stepsister. Sarah is also the half sister to Brandon since they share the same father but.
Step-siblings are children born of two different families who have been joined by marriage, defacto or otherwise, of at least one of their respective parents. A male step-sibling is a stepbrother and a female is a stepsister.
Then if they marry, they are sinning against God. Their marriage is prohibited. But if the stepsister and stepbrother are already married, then they should remain.