Thank you very much 8老師 for the help,
For 婆婆 case, I understand, and my husband will file I-864 and NVC will contact my husband for his mother's green card process.
For 公公 case, if he don't want to come here for the I-131 for two years,
-He does not need to stay in US for two years. He just needs to enter US to file I-131, then wait for about one month to tow months to conduct the fingerprinting, and he can leave.
is there any option that he can do in China?
-If he stays in China for > one year from the date the green card was approved without reentry permit, then his green card will be void. Thus, he may just abandon the green card in any one of the US Consulates in China, and then apply for the B-2 visa to travel to US.
Thanks again,