Yikes, people really don’t like Brendan’s stepmom. Her only onscreen appearance has her seeming pretty nice, and unless I missed something, they haven’t said that she broke up Eve and DoucheDad’s marriage. Even if she did, that’s more on him than her.
As for why she brought her son to the college visit, maybe she wanted him to see his older brother. Maybe she wanted to expand his horizons and help him learn to be comfortable in new surroundings. Maybe she wanted to bond with Brendan a little.
I work in autistic support, and I take my students everywhere. Stores, farms, museums, parks, etc. It’s part of helping them to cope with new places so that they can have different experiences. Just because they need more support doesn’t mean they don’t belong in certain spaces.