I think this probably needs legal advice, which you can get for free through Legal Aid. You just have to call and book a free consultation.
The complication is not being clear as to why the child was placed into foster care. It's not often the court will remove children from their parents' care because it violates the kids' fundamental legal rights, so there needs to be some serious and persuasive proof that the child's best interests cannot be met unless removed completely from their parents.
The other thing is that even if things have improved drastically, two years is not a very long time in the view of the court, so they'll be asking what steps have been taken to fix the issues, whether those actions have been effective, and why it is that the younger child is a focal point for proceedings, but not the older sister.
The other question that I'm inclined to ask is how long after the child's birth was the child removed from the mother's care, and what kind of relationship do they have now? How often do they spend time together?
We can't be given too much information about your case to assess it properly, because there's privacy laws that apply in all children's matters and the confidentiality agreement between client and solicitor won't exist, so I really strongly suggest seeing an accredited family law practitioner or at least a Legal Aid solicitor to get a better outlook of your options.
