Sister-in-law?

The first time I watched this film, it was definitely confusing. However, on the second run, I began to believe that they'd previously had a very deep connection simply as friends and relatives. Given Swanson's other horrid behavior, it might seem like he was kissing her just because he could, but I think it was a rare moment of tenderness for him. It was his way of saying goodbye, which she understood.

"YOU CAN'T PASS!!" - Lord of the Rings

My take is this. He is (rightly) attracted to her, because she is beautiful, smart, and together while his brother is in the 'looney bin' (his words). He is trying to shock her, provoke her, make her care about him and his opinion and his atrociously insensitive behavior towards both her husband (his own blood brother) and his dying father (we saw him poke his comatose Dad's forehead, after abusing his nurse).

He bullies her into coming onto his boat under pretext, then stops the motor. She is trapped what is she supposed to do? Jump and swim? Scream? Order him to behave? No. His bluff is called. He moves over to her side of the dinghy, strokes her hair and she doesn't push him away or react in a disturbed way. He moves in for a kiss...and she kisses him back! WOW! I did not see that coming! But to me it showed how incredibly vulnerable she was in that moment, and how unprepared he (Swanson/Heidecker) was to go any further than 'provoking'. He quickly moved away, turned the dinghy around, and delivered her back to the pier (and I'm sure he signed the papers without reading, he didn't care about the money or estate anyway but only the power he could use it for to bully his s-I-l).