Visitor Visa - B2

The last posting was divided into two parts. Now I have consolidated those into one and posted below:

I and my husband are US Citizens and residing in CA. My mother & brother (married), are in India and have a valid 10 yr B1/B2 visa. They visited us in 2011. Now again after a long gap, we wanted my brother and his family to visit us for a short duration (two or three weeks) during Christmas holidays 2019, we sent them an invite letter and suggested to apply for B2 visa for his family which includes his wife and two kids, that is they are my sister in law, nephew and niece. My nephew is a second year college student in Bangalore and my niece is a minor (14 yrs) and goes to school. So they were called for visa interview at Chennai consulate. My brother who already has a visa did not accompany them. Initially, while filling out the application forms for each one of them, it was mentioned that their intended arrival date in USA would be Sept 23. But later travel plans were changed to December, after we sent the invite. However, this change of plan was not corrected in the application form. Now during visa interview, the officer questioned them on many topics including their intended travel date. Along with their replies, they mentioned their intended visit to US would be during Christmas 2019. The officer never asked for any docs, which they were already carrying. And after the interview concluded, the officer said, your visa cannot be given and was refused on terms of 214 b. So what do you suggest on this? Is it a matter of concern that travel date correction should have been done earlier in the application form or?? Can they again go for a second interview in Oct/ Nov with the right travel date for visit during Christmas or is it too early to reapply again? I heard that at least a six month gap is required for reapplying, is it true? Any advise/ suggestions would be appreciated. Thank you, Sangita