Marcia's Daughter is Too Old
The big problem with "A Very Brady Christmas" is that Jessica Logan (Marcia's daughter) is too old. Marcia and Wally were married in 1981, that means the earliest Jessica could be born is in 1982, although there was no mention of Marcia getting pregnant in "The Brady Brides". Jessica Logan looks to be around 7 or 8 years old in "A Very Brady Christmas", so that means she would have been born in 1980 or 1981, so how could that be when Marcia and Wally were married in 1981? The math doesn't work... but then again, this is Hollywood where logic doesn't exist.
"I'd rather believe what I believe and be wrong than believe what you believe and be wrong."