My family is the same. Even with my second breastfed baby, they still question me. It gets worse because my cousin swapped to formula with her baby at 4 months because she thinks it's too much work and baby is too attached to her, and another cousin only tried a few days and decided she had "no milk at all."
What I've found about my family (or friends / stranger) when they doubt my milk supply:
(1) Because they can't see the actual milk, they don't know how much the baby has consumed, and they don't like the "unknown."
(2) Also, since breast milk is so much easier to digest, breastfed babies "eat" more frequently, therefore people believe that the mother is not producing enough to keep the baby full for a long time.
(3) Unlike formula, breast milk change in content/composition to meet the baby's need instead of increase in volume, my family doesn't understand why my kids eat so little (they see it in bottles when I expressed)
(4) Chinese like chubby babies! My family adore my cousin's chubby formula fed baby because he grows so fast and so big. When the baby's 9 months old, he weighed the same as my then 2 years old. (he's so chubby that he can't push himself up to crawl)
My advice: Believe in yourself and feel good about what you do. I'm sure all you want to do is give the best to your little one. If you want to keep breastfeeding, go for it. Breastfeeding is hard work, but also enjoyable and satisfying. Don't stress out about it because stress decrease milk supply!
P.S. I breastfed my daughter till 2 years old and now breastfeeding my 4 months old. It can be done (even with unsupportive family).