Uh...I think you didn't understand the main series' characterization of Marie if you've read it. She isn't aiming for a reverse harem just because she's sad and longing for big bro. Ever since she was back on Earth she was an evil and entitled and sorry for the word, b*tch, from the very start.

Even when she was a young girl she had the belief and POV that she is "special" and that she should be treated as such. That if things don't go her way she should just cry, and eventually somebody would fix the problem for her. And even then, after her unwilling brother---forced simply by the fact that he's the closest human being to her due to their sibling connection, deals with her problems for her she then just puts it off as "You should have already done that for me in the first place. You shouldn't have waited until I cried. You're so stupid."

When she played this game, she played it simply because she liked handsome men and wanted all the bourgeois the protagonist gets to experience in it, with zero consideration and none of the hardships that the protagonist has to go through to get all of those. This is precisely why she dumped the "playing and grinding" part of the entire game to her brother. Forced him to pay for the microtransactions, and even forced him to play to unlock all the scenes just so she can watch them later on (she still did after he died). She admits this in the main series once they finally recognize each other on the isekai world.

I don't think I need to remind readers, but the reason Leon died back on Earth was because he was so overworked from having to work for his own livelihood, as well as to support Marie who only wasted money and kept lying to their parents while she destroyed her own life being a literal B hanging out on lavish "paid by family" trips and basically sleeping around with "handsome" guys along with her fellow B friends. At the same time, Leon had to grind and pay for her games for her instead of resting during his free time, being threatened by her using lies towards their parents, heavily implied in the main series as well.

Heck, spoiler for the later volumes of the main novel series, she died on Earth some time after becoming a single mother and not wanting to support her own kid, chasing after "f*ck boys" like the guy that got her knocked up. Her parents had to force her to give up custody of her kid because they realized she would destroy her own kid's life and future given her entire horrible personality.

Going back to her being "lonely for big bro", she's not. She really isn't. She's using her past memories as an escape from reality because she was so used to somebody else (big bro) suffering for her and saving her from all hardships. That's why she developed that subconscious defensive mental tendency to call out for him whenever things got hard for her, instead of her actually trying to do things the right way herself. She's lonely for the days when everything was done for her and life was easy. Not for big bro. You could replace Leon with somebody else (like dad or mom) who would save her hide for her all the time, and she would probably be calling out for them instead.

All honesty, I don't see the point of this spinoff existing. It's comedy for the sake of comedy by focusing on the stupid characters from the main series. Given Marie doesn't change at all even after she got married to the guys in the main series and was still mooching off of Leon by that point, we're not going to see any character progress here other than more glimpses into her self-convincing mindset and the tiny bits of what could have made her a better human being. I would've been more interested if they actually poured more resource into the main series, or at least made the spinoff be about Luxion of the past when the "Old Humanity" still existed in the isekai world, since that entire part of the plot was not included in the VN Leon played for Marie until he died. It's an entire story outside of the game that never seems to have gotten actually explored. But hey, I guess we have another manga to have a laugh with.

yeah Selfish is the word that best describes her. I'm glad to see someone who also sees her that way