Wow- ok, I didn't see your first email, but here goes:
If this is just your boyfriend's family- well, there is no ring on your finger, sweetie. Of course you want to be POLITE to them, and it sounds like you have tried your best to do so. But seriously, let it go. These people are not 'your' family if you aren't married.
Even if you do get married, you will NEVER be able to please some people. All you can do is KNOW that you have behaved politely and tried to be nice- and let it go. I know from personal experience how hard that is.
But if you don't let it go and keep worrying about it, then you will just make yourself crazy and it will hurt your relationship with your BF as well. I don't know how HE has handled it- but he needs to pick up the ball from now on and deal with HIS family.
I am not saying either of you should be rude or mean, but if I were you, I would have a talk with your BF. Say that you understand this is his mother and he loves her etc. but that you have tried to be nice and his mom just doesn't seem interested in that.
If he wants to go see her, fine, but you should not have to be subjected to nastiness and don't need to go. If something really needs to be 'said' about his family's behavior, he is the one who needs to man up and say it.
But you're right- good for you for recognizing that it's hard and he IS in the middle. Just remember- YOUR little family is what is #1, most important here. Relatives can create all kinds of drama, its true. But as long as you, your BF and your son are good and all together, THAT is the most important thing.
Just have your family over if they are loving and supportive of you and focus on that. Be polite if you talk to your BF's mom or sister, but I would not feel obligated to go out of my way to invite them to things or call them up if I was you. Above all, don't let their attitude affect you- be strong and focus on the people who love you! Good luck and God bless!