Sharlene,
I suppose girls are really more petty than guys and they tend to question our intentions and doings even tho they don't fork out a single cent. That's why I don't sought my girl friends' opinion on my wedding preps.
Honestly, from what you say, I would not include her as a key person in my wedding. She sounds like she is someone who could make some curt comments like what some jiemeis did and spoil your day. I've always thought that elder people thought it unauspicious to have a married woman to be a jiemei?
I suppose we're more particular abt our wedding than anybody else since it's once-in-a-lifetime thing for us but we are always attending wedding dinner so it feels like wedding dinner's a dread. Well, if I were you, I'll probably list down the duties of what she's gonna do and brief her face-to-face on it, like,"Eh, as a jiemei, you got duties leh ..." and tell it to her one by one based on procedures. See if her facial expression changes and if displeasure shows, ask her if she's comfortable with it or if she would prefer to spend the bulk of her time with her baby. If you're best friends, it shouldn't be too difficult to talk the issue out, or simply ask her,"Do you mind if I get another person to SUBSTITUTE for you?" ("Substitute" sounds like she's really wanted but have to give her up). Give her reasons like too many jie meis or your mother worries that she can't cope with a baby and ushering la. I really can't see how she can carry a baby and usher the guests at the same time. Trust me, you can't.
Worse comes to worst, let her know you really don't want her to juggle her duty and her baby on your wedding day - "It's too much to ask of you!"