So what do I say to her when she calls? Should I invite these relatives or not?
Oh yeah and the party is less than 2 weeks away.
I don't think a single etiquettishly-correct answer is set in stone here. I think it's your call. I think it depends on various things.
Is it the kind of party where exact numbers don't much matter - eg a large venue, a buffet meal, no seating plan? If you can't accommodate, or afford, any more guests, then the answer is No. If you can, then it's up to you.
Do you know the relatives your MIL is talking about? If you've never met them, and DH hasn't seen them in years, it seems pointless to invite them. If, otoh, they're relatives you think he'd be pleased to see but just didn't think were *important*, then... it's up to you.
How well do you get on with your MIL? What kind of person is she? If she's prone to interfering and overriding your decisions, then the answer is No. If she's generally pleasant, and in this case is anxious that your DH has a good time and that everything be done properly, then... it's up to you, but I'd be inclined to say Yes. If your natural response to MIL would be "Oh, Mum, I should have asked you sooner! Thanks for letting me know," then you probably want to invite them. If your natural response would be 'I wish she'd stop interfering' muttered under your breath, then you most likely don't!
I know that there are all kinds of etiquette problems with "B-list" invitations etc, but I think the world in general is less worried about the formalities. If you decide to invite more people, you could call them, or write a quick note explaining that you hadn't been sure whom to invite, and hope they won't be offended at the short notice. If you do not wish to invite them, explain to your MIL that it isn't possible to add to the guest list at this stage.