As long as you know what's best, do it and try not to let the criticism get to you! I was in a similar situation-- my mom was aghast that I don't use formula or bottles (she believes I won't have enough milk), and my dad thinks I'm compromising the health of their granddaughter (he believes formula is best). My mom would even talk to other aunties and relatives, and try to get them to talk me out of breastfeeding. Once she saw I wasn't going to give up, she started to try to persuade me to stop by the time baby is 3 months, even having a nurse come to tell me that 3 months is enough.
Now dd is 6 months, and we're still happily nursing. My mother still makes comments like "see, your daughter doesn't want your milk" when dd is too fussy to nurse, or tell other people at the dining table "she's afraid her baby will stop breastfeeding, that's why she doesn't give her solid food", and even said to me, "you are inhibiting your baby's growth."
I used to get sooooooooooooo mad, and was once furious enough to tell her to shut up. It didn't help anything, and I regret my outburst.
I just look at my daughter and feel grateful that we do have a nursing relationship. I know I'm doing my best for her and I'm comfortable enough now to not care about what anybody says. It was tough at first though. Keep it up and good luck!!!