Hi, all!
I checked with my sister (who lives 5 min away from parents) and was told, mom's concern, after all, does not appear anything to do with the air travel for baby@18months.
Now that she told me, I remember my parents and my sister/brother-in-law, all of them sick during a holiday season last year. When I video-chat with them last year, all of them had that nasty voice from cold, and they didn't look soo well.

I suspect that my mom is worried that if they become sick again like last year, despite all the efforts to avoid becoming sick, prior to and/or while we are visiting them, mom feels we won't be able to enjoy the time together. (well, yeah).
Besides, my parents home is not built with a "central air system" like in the U.S., I believe mom is worried that my son may become sick while he is at the grandparents house. Major reason why she wants to push it to next May. I kind of understand this now....

This is tough question now. I'll really have to discuss this with mom and my husband.
Will report back!