回複：visa to US, urgent question

better not say your parents-in-law are not in good health. otherwise, the VO may think they'll be trouble of USA. and VO would also think that they'll stay in US because of the better medical conditions here.
Find another reason, e.g., your parents get used to traveling with your sister-in-law.
since your sister has two houses, it's helpful to approve that she doesn't have the immigration intent. also the employment letter of her, and some evidences showing she has deep feeling with her daughter and hu*****and, which can help to show that she wouldn't give them up.
but all will depend on their luck. there is no standard.