The thing is, she is not MC's sister; she is her main love interest. She is practically using him and emotionally blackmailing him. The developer says they are good friends and have always helped each other in the past. Even mentioned in game, but after people make up their minds about her, she helped him when he needed it and he helped her when she needed it; and she was with her boyfriend out of necessity, not love; but it was too late for the decision. So, it matters because we can make educated decisions about her; we would have a positive bias about her. Up until the decision point, she did not even deserve to be heard after she mentioned money.
For example, MC mentioned he lost his car and had to cycle to work every day, and she didn't even know about it. If we know the background of it, this could swing both ways. If she didn't even notice MC cycling to work, if she didn't even bother to ask, then she is not a friend, just a leech trying to suck money off of MC for her boyfriend's debts while she knew MC was in love with her. Or maybe she dug; she wanted to know, but MC did not tell her out of his pride. She might have tried to help him, tried to give him some money for the bus when it rained, but MC refused. You cannot blame her for this, can you? So like I said without background of events, things can swing both ways.
Now, when you try to tell a story and you are vague about something, you are risking having readers fill in the gaps with their own ideas. When they fill the gaps, it's nearly impossible to change them. If this is about some side character, it won't matter too much and even can add some depth to the story, but if you are doing it for the main focused character, it is an incredible risk. Clearly, dev doesn't want us to fill those gaps with negative things, so it is important that dev fill these gaps themselves. That's why that matters.
Like you said, if you see your sister in Nicki, you fill these gaps positively; if someone else sees their toxic ex, they fill the gaps negatively; if someone like me, who approaches everything with a grain of salt, expects the worst from her, and so on. Leaving some plot points vague and letting readers fill them is an extremely good branching method; you let people decide what they think about certain people, then branch accordingly, but if you want people to think a certain way, you should tell them. How you do that would be your writing skill, but you have to.
Some people might see 'rape' category or read what the patch adds and think "oh no..." and not realize the game can basically be as wholesome and innocent as you want it to, really.
