I think you can still get a happy ending anyway, which is what's important.
Not as happy as the same outcome (save parents) when you spare the Nightsong, I'd say. But I might be reading too much into things with this.
At the point where she's about to turn on Shar on her own accord - for you - you instead push her to remain loyal - push her away from you (eventhough you don't really know that at the time, she does, it's why she's been doubting about becoming a Dark Justiciar in the first place). Because of this, neither of the chats about being together forever after happen (the one straight after Nightsong & the one when entering the Lower City in act 3). It makes the epilogue dialogue when saving her parents make a lot more sense; she's never confirmed with you that you're actually planning to stay with her. All she knows is that when she was ready to devote herself to you, you instead made her devote herself to Shar.
Unless this chat happens after you save her parents as Dark Justiciar, I don't actually know about that.
