I don't know, I think an "Are you freaking nuts!?" would be a legitimate response in this situation.
I agree that the MIL is probably already doing more than she really wants to, but is pitching in because it's family, and because they are in a tight spot - losing a job and having to take a much lower paying one can do that even to good planners. But there's a huge different in pitching in to help, at a disadvantage to yourself, when life happens, and doing so because of people's deliberate decisions. If MIL were babysitting by choice, and the LW asked about her willingness to take on a second child, and MIL said great, that would be a totally different (and more responsible) situation.
As it is, as far as I can tell, the LW's plans are "Oh, I'm sure things will get better for us in the future", without any actual evidence or concrete plans for this.
If your current ability to support your family is dependent on someone else's generosity, you don't deliberately add to the family size, and you are very careful with birth control. And I do think that anybody who is helping to pay for your current family's support gets a say on whether you have another.