To be fair, Mia only had a ~50 day update back then and was always meant to get another one for herself. Even if one excludes pregnancies etc. Eve's update will certainly have double the amount of time invested and thus certainly get more story/content.
Btw., right out of Discord on the topic of Eve's update:
Yes, but last I heard of Mia's second update, it was phrased in a way that put her character to the wayside again.
Furthermore, double the amount of time invested doesn't help anything when it's split between two other girls and then the potential "trap" thing which is going to take a deal of time for writing, dialogue, art, posing, etc.
Now if it was just the sister and Eve without the whole trap thing it'd be far less of an issue IMO.
For instance, while Roxxy's update had other girls, it was primarily just her. That's how I would think an update revolving around a character should be.