A.~
Do what feels right to you and for your baby. If he is weighing in okay at the pediatrician, doesn't appear to be malnourished or as if you are not taking care of him~ then you are just fine.
I also waited until six months before starting solid foods. I did it as I wanted to avoid any possible food allergies and I wanted to be sure my son was ready for it, too.
When someone sticks their nose into your life~ you have to put your foot down and make sure you go with your instincts. Just because you are a new mom, it does not mean you are not without resources or alone~ It does not mean you will screw anything up and it does not mean your sister knows more than you do.
If you need to, you can supplement some formula with the breast milk. (I researched and found I a brand I was comfortable with, and we had no issues.). Just nurse first!
You will need to tell your sister to back off~ this is your child, not hers. The stress of her interference could be affecting your milk production as stress does decrease milk supply. So relax and go with your gut.
Keep doing what you are doing ~ I also think it's wonderful that your hubby is supporting you, too. :)
If your sister has children, tell her that all children develop and grow at different rates and if she has a set 'rate' in mind with your son, you would appreciate it if she stopped comparing. (That has been a big pet-peeve in my family. I despise my son being compared to my nephew - especially as my son was a preemie by six weeks.)