DITTO! PS stated it simply and well.
He’s none of the above relationships. He’s capable of falling in love with his SIL…doesn’t necessarily mean that he’ll act upon those feelings in a strong manner (such as deliberately splitting up his bhabi and bro). And cousins can fall for each other…even though one might say that they are “LIKE” brother and sister.
Strangely though…this thread is very similar to Sara_Pathan’s recent thread about a BIL falling for her cousin-bhabi.