Noone cares about your sisconning ways, you perv! Mommy always comes first! :relievedface:
But its true, if we are talking about said girl as love interest it is advisable to make it on par with any other love interests. So if mom would be romancable and the girl too, they should be equally satisfying in terms of story, character- and relationship developement. I'd say its quite a challenge though if the Dev is biased. But thats the reason why it is always important to develop a game along his own kinks, at least at first, because that way fake choices are avoidable. Its bad when a Dev advertizes two routes that are supposedly equal and then end up favouring one heavily over the other.
And i definitely agree that make the girl pretty please, interesting! And have a personality that befits a 18 years old.. Oh how i wish to see young adults being.. young adults. Actually i'm into underaged but when we are talking about a 18 yrs. old one who is (i'd assume) starting university this whole 10yrs. old attitude isn't all that appealing i so often see.
A pretty girl at that age will surely have a boyfriend too. Or girlfriend. So all this "i never had a boyfriend" thing is.. meh. I'm pretty much on the same page as the vile sisconner above
that this doesn't have to be supereasy with the girl, whether she is a sis or not. Especially not in case of a main love interest. Of course its all depends on the setup. In a harem game, things shouldn't focus on a single girl too much. But in a game that has only two love interest, well, those should be a tad bit more interesting, intriguing and hard to get than the rest. But this can happen in different ways.
Sex for example can happen early on. A relationship doesn't have to develop in a linear fashion. Girl can change her mind. Catch MC with someone else and absolution can come at a price or only after quite a while. It all depends whether you wish to do a light hearted rompy story or something with more shades of grey.