In that case, tell them it is unlikely that they will be sponsoring your wife and their daughter at the same time. For USC/LPR unmarried children over 21 years old, it would take around 8 years or so to even get a visa interview. Enough time for your wife to move to the US and eventually become a US citizen, if she's interested. When she becomes a USC, your and the joint sponsor's I-864 obligations end. So, assuming your wife pursues US citizenship as soon as possible, her joint sponsor will no longer be financially responsible for her by the time their I-864 for their daughter takes effect.