How can I stop unwanted potty training advice from mother-in-law?

My husband was adopted at age 11 by his adoptive sister (she is actually his cousin). His adoptive mother is in all essence my mother in law and the grandmother figure to my children ages 2.5 and 1. She has never been married, has never had a boyfriend, and has never had a child of her own. She is in the military and can be very aggressive with her communication and has a tendency to believe she is the expert in ALL areas. I am a stay at home Mom and am quite sick of her making comments that my son (who will be 2.5 in two weeks) is not potty trained. I want to tell her to knock it off without being disrespectful but honestly she takes me even challenging her as disrespectful. I have done many things to encourage the potty training process with my son. We read books about going potty, we watch the Elmo potty DVD together, he watches both his father and myself use the potty, we have talked about being a big boy and wearing big boy underwear, I take him to the potty when I see cues that he needs to go and encourage him with rewards and praise when he does go. He was doing great for a few weeks and now is not interested. When we talk about the potty he always says "no". He also told me he wants to wear a diaper like his baby sister. I have my own strategies for how to deal with this and will never resort to pressuring him. My son knows all the phonetic sounds for each letter, can recognize each letter, and is now starting to even write some of the letters. (she never says a word about this) He just isn't into the potty training thing and until he is over 3 I am not really all that concerned either. How can I get this woman to back off? She can't even change a diaper and honestly I dread her coming over here because I can't keep my cool any longer. Her sister (who also does not have children) is the same way. HELP!!! I need some ideas on how to communicate with her.