Serious relationships, marriage, pregnancy, sharing finances, etc. all can come with extremely stressful times. Grace is 'mature' in the sense of she could move out, keep a house organised, feed herself, pay the bills but she doesn't strike me 'mature' in the sense she could cope with those difficult or sensitive situations.
Yes, I do assume that this relationship will turn serious very quickly and will follow in her siblings footsteps within 2 years max, but I seriously hope she takes it slow and doesn't mould herself in to someone she's not to please a partner to avoid these difficult times and potentially a break up.