I think it's amusing that your MIL wants all the kids together, but won't pay for your DH to actually make her dream come true. But under the circumstances, you should view these "invitations" as "solicited gifts" to your MIL. So you can all be a "family", you give MIL your attendance at Cirque for her Christmas gift. And that's it. It's not her gift to you, it's your (and your DH's) gift to her. I think it's a lovely change from "stuff".
And every time she poses one of these offers, think of it as your MIL asking for a present, and act accordingly.
But since MIL gets everyone else's tickets ahead of time, often the OP and her DH can't even sit with MIL and the rest of the family. So it's not like MIL even gets to enjoy their company; they're seated separately from the group. Because of her own practice of buying/reserving a clump of seats ahead of time that doesn't include OP and her DH.
I agree with the PPs who feel that the whole thing sounds like a power play. MIL seems to be going out of her way to make the OP and her DH feel unwelcome and rub in their faces how inclusive she is with everybody else. Added to her behavior before their marriage of paying for her son's ticket but not the OP's, it sounds like now she's punishing DH for marrying OP; she's showing them both that they're persona(e) non grata.