I'm Lucy, and I'd be happy to answer your questions today. I'm sorry to hear about your situation.
It is a conflict of interest for a lawyer to represent anyone related to someone who works at the firm, so he could not work for you as a lawyer if your ex-husband's sister had already been working for him. However, a lawyer owes a duty to existing clients, both of competence and of loyalty, and your lawyer has breached those obligations. You can actually sue him for malpractice, and your damages would include the cost of paying another lawyer to help you get your children back after your lawyer wrongly advised you to sign a paper giving up custody. The statute of limitations for filing a lawsuit is two years, so you can wait to see if the other case works out, if you want, but you don't have to. Fl. Stat., Section 95.11.
Your lawyer also needs to return any money you paid him that he did not earn, and you can include a breach of contract complaint when you sue him if he does not account for all the monies and prove that he earned them. You can also insist that he return your file to you, because that belongs to you and you'll need to give it to your new lawyer.
The time for filing a Complaint against a lawyer with the Florida Bar association is 6 years, so you have plenty of time to do that. See Rule 3-7.16. Here are the instructions for filing a Complaint:
If you have any questions or concerns about my response, please reply WITHOUT RATING. It's important that you are 100% satisfied with my courtesy and professionalism. Otherwise, please rate my service positively so I am paid for the time I spend answering questions. If you are on a mobile device, you may need to scroll to the right. There is no charge for follow-up questions. Thank you.