I understand you’re upset; I would be. But, if he tied up his own dog in the back yard, I can see how he would think this is acceptable…unless he knows better and is just being a turd…it’s how to say without knowing the guy. I wouldn’t let it get in between you and your sister though. She might not even know about it.
My brother is exactly the same way. When I go over he suggests I put her in the back yard, which isn’t landscaped yet and has a questionable fence. I’d rather leave her in the car (not in the heat of course). He doesn’t mean to be rude, but sometimes I wonder…
