SIL said her partner told her that her breasts were his and sexual not food.
Stare at her for fifteen seconds and then say "I'm sorry, I must have phrased something badly. I didn't mean to imply that I wanted your opinion on how I feed [DD]."
Or reply "Gosh, poor you. I'm glad DH is more considerate and supportive than that."
Or "Poor you -- I suppose we can't all feel secure and confident in our own femininity, but you're putting a very brave face on it." Then whenever she protests about how great her sex life/bottlefeeding/etc. is just pat her on the shoulder and say "That's the spirit! I really admire the way you can put a positive spin on negative experiences..."
Or if you want to sink to her level you can confidentially say "Actually, [DH] really likes how much bigger [or "firmer" if they aren't really bigger] my breasts are now I'm bf and wants me to keep it up for as long as possible. He feels quite sorry for [her DP] that he missed out on that..."
Is she your DH's sister? If so maybe look serious and say "SIL, it's very considerate of you to take such an interest in your brother's sex life, but it's also more than a little creepy. Do you think you have some issues you need to resolve?"
Of course, I'd actually say something a lot ruder than that, if it were me... 
P.S. Women have every right to bottlefeed if they want and inform themselves, and I'd never say any of these to anyone who wasn't behaving like a git over the whole subject. But your SIL needs a good slap.