My Aunt was similar--though not a JW. My Dad used to say, "I know she is a little extreme, but she means well."
Just remember, this lady is terrified for your child. She is obviously in a panic. She is also on the verge of going off the deep end, from the way it appears. Don't be surprised if a breakdown occurs.
Maybe you could send a letter back to her, being as kind as possible. That you appreciate her concern, but that this is a matter between you and your husband. Maybe you could ask how it came to be that she feels she has the authority to send you such a letter asking these questions? Have they changed the rules on women becoming elders?
Tell her that you love her, but that this matter is personal and does not involve her input.
Express concern about her mental stability and tell her that you will pray for her.
If she continues to harrass you, and you perceive to be a threat, I agree with the others that you should consider taking legal action. Your first concern is your child. You just don't know what someone in that state of mind will do. Aggitating or arguing with her may not be the best thing. Try to be kind, but firm. POlitely let her know that it is none of her business.