Please don't pretend excitement or happiness for her. Tell her honestly in a caring way why you are not excited and happy for her. Ask her some direct questions about her goals and what she wants for herself and her children and then ask her if marriage to this man will give her those things. Then ask her how she knows that the marriage will give her what she wants as well as what she needs.
This is the sort of conversation that will need to be had over time. Many people do not have goals nor do they know what they need. It takes time to think them thru. Perhaps she isn't mature enough to want to consider goals and needs. If she doesn't want to do this exercise you'll just have to let her go to make her mistake. But that doesn't mean you need to act as if you're happy and excited about her choice.
You can be her friend and stand beside her without agreeing with her choices. Honesty is always the best policy. If you're fake about this she may sense it. Even if she seems unaware you'll know you're dishonest. That sort of action causes stress in a relationship.
So, talk with her in a kind way as long as she's willing to listen. If she doesn't want to hear it back off but remain true to yourself. You're a better friend when you're own best friend.
I'm concerned about her children. Forming a step family takes alot of sensitivity and work by the parents. She has at least one teen and in general teens are having enough difficulty adjusting to being a teen that a step-parent has a difficult time with them. And a new baby before they've built their own relationship spells disaster. Has she considered any of this?
If she's a reader and willing to learn there are many good books about relationships that could help her make a better decision.
From your description she sounds very immature and focused more on her immediate happiness than on long term goals. It also sounds like she's not considering the welfare of her children. If this is the case you probably will not be able to have a good discussion about her choice.
In that case, I'd approach talking with her by telling her that you are concerned about both her and her children and that because you love her and want the best for her you cannot be happy or excited. Then wait and see what her response is. If it's denial or anger I'd not try to talk anymore with her.
The mother who suggested that remaining involved in her life could create chaos and drama that will disrupt your own family is asking a good question. Consider how this friendship affects your life and decide if remaining in her life is good for you and your family. You can remain willing to be her friend without taking part in her drama. This could mean not being involved in her wedding plans and spending very little time with her. Remember that your happiness and the happiness of your family should be your top priority.