Red flag, don't go until he's grown up. Unless you're going with more family and not yourself and your son
I wouldn’t let my two year old stay alone with ANYONE. I would simply say, “Thank you for noticing that I’m interesting and a first time mom. We’d love to bring him by to see you while we’re there, but I’m not letting my baby stay anywhere by himself until he can clearly communicate and understand things better. I appreciate you supporting me in this as a first time mom.” And gently smile and let it be. *hugs* I can’t think of any caring mom that would be okay with that. 😅
Maybe don’t take the trip. She doesn’t know you in person and she’s trying to cause trouble. But even if you did go she can’t keep him. It’s your child.