I know that blueyz has already replied but I just wanted to clarify one thing.
There is a difference between offering to cook for a certain number and then rescinding the offer if that number grows, vs. saying something like "I'll cook the food but only if you cut your guest list."
If the current guest list consists of 11 and blueyz says "I'll cook" and the list grows to 30, then she has the right to say, "hold on, I was willing to cook for 11 but not for 30, sorry."
But if she already knows it's going to be 30, or has a pretty good idea that it's going to grow to that, I think it's best not to offer in the first place.
This far in advance of Thanksgiving, especially given the SIL's change in circumstances somewhat recently, I don't think it's unreasonable to assume that she hasn't firmly decided on how many people she's inviting. So if this was a conversation happening a few weeks before the holiday, and SIL had either indicated she was sticking with her previous style of holiday or hadn't said one way or another, I'd agree with you.
But with the complaints about not having room/money, and with the fact that TG is six months away, I don't think it's unreasonable to assume that she's still deciding what the scale of her hosting will be. In that case, blueyzca01 letting her know that if she scales it down she'll get help with cooking seems like a thoughtful gesture, since it's information that may affect SIL's decision. It may not change her mind, but it's still relevant information at this moment. If she decides to stick with the larger guest list, it would be rude and counterproductive for blueyzca01 to try to convince her to scale back, and better to just plan on either showing up as a guest or skipping the event entirely.
Either way, it's not blueyzca's decision to make or problem to solve, and she shouldn't feel obligated to listen to repeated complaints about the same thing.