Princess,
I, understand where you are comming from sweetheart, and like danielle I too would give a limb to be able to spend 5 more minutes with my mother, And I'm sorry for your loss danielle.
Now, I don't understand why your boyfriend is unable to attend with you for moral support? She has to know that you tell him things and that if he remains in your life he already knows about the wedding,so why is he not allowed to attend?
If you have already spoken to your mother and she knows how you feel about her new husband to be then your attendence shows that eventhough you don't approve you are still a big enough person to be there for her and stand by her through good and bad times. She's an adult and I'm sure quite capable of making her own choices, good or bad, but she's also your mother and needs your support.
With that said if you still feel that by your attending this wedding you will be giving the impression that you approve then speak your mind! Let him know that you are there souly out of respect, support and love for your mother and not for him. Good Luck to you whatever you decide hun. And should you still decide not to go, that doesn't make you a bad child, or person.