really, you don't have to justify yourself to anyone, changing the subject after making it clear that it is not up for discussion is a good idea, if you can't/don't want to get into it. You could always bring up the new AAP recommendations that say you should go at least one year, and the WHO says at least 2.
Really though, my advice is to figure out a way to get it in your own life that you feel good about it and even proud of providing your child with breastmilk/comfort/warmth/security and don't feel like you have to justify it to anyone, including your own mother. Get to the point where her asking about it is more annoying to you than you breastfeeding a 13 month old is to her. Make her be the one to feel uncomfortable with asking, not you the one uncomfortable with doing. She got to make decisions about parenting with you, now you get to make decisions about parenting for your children.