Why did Aunt Edna like Eddie but not Clark?

Well I think often in families the favorite child is often not the most successful or even the kindest. I think often the favorite is the neediest because it allows the older generation to still feel needed whereas the more successful have found their own way in the world often far from home. So in short I think she liked Cousin Eddie because he was an extension of Catherine who stayed close to Aunt Edna and Clark was an extension of Ellen who was far away.

I have some family members who just don’t like anyone. Or for no reason they single them out for no reason.

I have an aunt who one Christmas called all my female cousins and I to sit down and open an early Christmas gift. We were all excited. She passed out cute little handmade gift bags and all the girl cousins got one. She handed me an envelope. She then sat down and told us all on the count of three to open them. I already felt embarrassed and left out since I had just a plain envelope so I watched my older and younger cousins open their bags. Inside was a hundred dollar bill! I quickly opened my envelope expecting of course money. I had a thin note pad with a cat design on it. I was confused and held it up. I muttered thanks. She went on to tell all my girl cousins to spend on something fun. She didn’t even look at me and not one person in the room said a word. Not even the adults. I was 10 years old. My point is this aunt always treated me like this. No reason why either. Sadly my parents never stuck up for me so I just had to accept in her eyes I wasn’t worth her time.

The Riff-Raff like each other.

It's like working class people trusting each other, but not some corporate suit. Nothing is more riff-raff than Eddie, besides Edna - even their names are similar.

Could be a plot hole because I can't think of a good reason why

I don't think the movie is that deep, but Eddie barely passes as a civilized man. Crossing him would only bring more pain and suffering on Aunt Edna. Clark is a man that can be relied on, and she takes advantage of it.