Oh I agree, Bella is definitely a bad person, manipulating people and screwing over her own sister. But I can't say that Elaine is much too better, sure she tried to be nice to the MC, but only as long as it didn't jeopardize her access to all the fine things in life that she got by being a psychopath's trophy girlfriend. She knew it was wrong for Donald to treat the MC like that, but she went along with it anyways. Arguably, that makes her more morally culpable than if she simply didn't give af about the MC.
From a neutral perspective, I'd say both sisters are terrible and the money is better off going to an animal charity than either of them. But looking at it from the MC's perspective, Bella was always nice to him and even protected him from Elaine when he was younger, whereas Elaine was always a bitch to him and threw him out on his ass. So I can see why the MC would be partial towards Bella.