I guess the bigger question is how does your fiancee react when these things happen. Does he appologize or rationalize for her and expect you to let the "punches" roll off, or does he support you by stepping in and introducing you when you're left out of introductions, including you in conversations when she talks around you, etc. Has he had a frank, private talk with his parents about his feelings for you and that he wants them to respect you as his future wife and mother to all six of the children? Remember, his role between the two of you is a tough one--he loves both his mom and you and doesn't want either to be angry with him. That is not to apologize for him, just to have you understand he is also in a tough spot. Still, he is to be your husband, and the bond between the two of you and with your children is the primary bond--not the one with his mother. He is the one to make that point perfectly clear to her.
Another question. Do you have a ring and a date? Often parents such as his mother will not accept the new fiancee unless they know it is serious, particularly if she liked his wife and felt betrayed by her leaving the marriage and treating "her boy" that way. If she treated the wife the same way, you can be sure she isn't going to change her approach to you any time soon!
Being hurt rather than standing up for yourself and your children only continues to hurt both you and the kids--certainly not her. Being defensive doesn't work either, because clearly this woman has a practiced, well-refined cutting edge on her tongue and knows exactly how to ice you out. Next time she omits you in introductions when you're out without fiancee, and before the conversation has a chance to move beyond the introductions, introduce yourself by saying, "And I'm [your name], [finacee's name]fiancee, how nice to meet you." How about some questions back to her when she makes her cutting remarks, like, "Why would you SAY [slight emphasis, not overdone] such a thing?" "You really MEAN [ditto on emphasis] that?" "Well, that's YOUR [ditto] opinion and I see it differently." In all cases, don't say another word after that remark. Let her deal with the comment on her own. Questions such as these when she starts criticising you about the kids are also appropriate, especially if the kids are present. They may not say anything, but you can be sure they are getting hurt by such critical remarks about their "never quite right" performance. If they see you standing up to her by asking these questions, they'll learn a valuable lesson in how to treat verbal bullies themselves and know you support them 100%.
Just remember, this is your fiancee's mom, and you'll be dealing with her for the rest of your life when you're married, so no reason to make an enemy of her, AND no reason to be her door mat, either. Your kindness, self-intervention, and attitude will make the situation easier to manage, but it isn't going away. Ever watch Everybody loves Raymond? There are people like Marie in this world, and you can't let them dominate the place in your life that is reserved for your fiancee and your children and your lives together. Good luck!