Mizuho seems like a terrible girlfriend; she's lazy, flighty, uncaring, outright ignores issues when they come up, and is only with Ena because Ena said that she loved her. So now the only question is whether this story will be about Kana trying to make her sister actually give a damn about her relationship or if it'll be about Ena leaving Mizuho for her sister.
Speaking as someone who has been in two relationships that were just like you describe (though throw in therapy and an inability to accept any sort accountability) i think you have the right idea, I did allot of things like Ena does with wanting to help out and just doing nice things for no other reason then because I wanted to do something nice and it is pretty frustrating when the other person just can't empathize with other people at all. I should add that Mizuho seems very insecure about the whole situation and somewhat of a recluse (I think she's only worried about what people at work will think, i don't recall any mention about her "friends" or "family" getting ideas about them.) which further makes her out to be someone who just can't empathize with other people. Not sure how much this manga is based off real-life experience's of the author but it strikes an awful lot of similarity to my dating experiences with people who were just way to self-absorbed to be in a relationship.