I answered below before reading this. To me, it sounds like she became a mom waaa-aay before she was ready for it, and she's projecting her frustrations and regrets onto you. I can understand it, but your situation is so different. What I would do is respond to her that you appreciate her concern but you have given it a lot of thought, you are in a much different situation than she was when she became a mom, and that you your mind is made up and you'd rather not have her rain on your parade. You need people to be excited for you. I think she will show support or at least back off. And if she doesn't, I think she definitely will when your son has her wrapped around his little finger. Someday, she'll thank you for forging ahead with your adoption despite her misgivings. (I've heard this many, many times from people whose families were dead set against the adoption, not just concerned. Those little buggers can really grab ahold of hearts and bring out the best in people.)
Sorry if she "gets" to you. Some people just have that knack. But set your boundaries, then look elsewhere for support for now. You SHOULD be excited! You DESERVE to share your joy with people who will rejoice with you!