This will end up very badly because Gid is very overprotective about his daughters, and he hates seem them in pain because Momo will be crushed that is to be sure, and Lala might even feel it worse than her.
I am starting to get in a mood to bash Rito right now in the current chapter Nemesis did the right thing in making this come out in the open but like a typical wuss that he is the guy can't accept the Harem if that was the case why in the hell you kept Lala in your Home.
Rito is a selfish bastard and a wimp it is stated in the story that he kept Lala as a insurance policy if things don't work out between him and Haruna I wonder if he truly loves Lala because Haruna is a nice girl but in terms of personality Lala simply outclasses her in all departments.
Haruna is the only person that can resolve this problem if she is a good person like I think she could literally force Rito to accept the Harem by saying that if you don't accept Lala as the second girl in the equation I will not be in a relationship with you anymore.
Rito in my opinion is not designed for Harems he doesn't have the personality and traits to handle this I actually thought that Ichika from Infinite Stratos is worse than him but now I am not too sure.