This game looks good. While the art style is not my preference, it looks professional and coherent. The story and gameplay are really simple, though. There is no scaling in difficulty whatsoever. Summons stay at the low price of 100, they don't fail, and the money for a summon can be earned with the first girl, from a single sex act. Training girls unlocks new scenes, but doesn't impact gameplay. Since earnings go up based on the order of summon, with the last girl earning most, it's all too easy to summon until you run out of cash, perform a single sex act with the last girl you summoned, and repeat until you have collected all girls.
Training costs also stay the same, so once you have the last girl you can easily max out all training.
I would suggest that summons can be earned through achievements and story-driven events, to give the player a goal. The money earned can go to upgrade items and maybe daily wages for the girls - adding an actual management element. Think of upgrades broadly - you might purchase events or items that unlock sex acts.
Finally, if there is training there should be stats which impact gameplay.
Happiness, stamina, sex-appeal, sexual ability and eagerness could all be stats that impact a girl's financial performance and add gameplay elements.
Especially if certain decisions or game mechanics can also decrease these values, you'll have yourself a proper brother management sim.
Great work so far, though - I really enjoyed the little training scenes and the look and feel of the game.
Keep up the good work.