Please understand that dogs are considered property, just like a chair or a table or a television. Also, possession of property creates a presumption of ownership. As such, there is really nothing to sue her for in this situation.
The chip presents an interesting situation. If you have requested that she put the chip in your name and she has failed/refused to cooperate, then you could consider a small claims lawsuit requesting assistance from the court to get her to do so. My concern though is that forcing the issue may not be prudent.
While I do understand your concern about "sit around and worry", if you do nothing, she may do the same. The net result is just what you desire....the dog remains with you. Conversely, if you take a proactive stance and file a lawsuit, this may force her to respond in kind and file a lawsuit against you. While my speculation is that you would prevail, it is just that, speculation. We must acknowledge that it is possible you would lose and the dog would then be taken from you.
CAN you be proactive and take the matter to court? Sure. SHOULD you be proactive? That's a tougher question and one that can only be answered by you. If, understanding the risks, you still prefer the proactive approach, then proceed accordingly.
Also, several customers have asked how they may direct a question to me in particular. If you specifically want me to assist you in your legal matter, just put "FOR JOSEPH" in the subject line and I will gladly pick up the question as soon as I am on-line.