How to use complex conditional to fill column by cell in pandas in a efficient way?

If the available destino_heranca always appears first in each num_familia_raw, then you can do a transform:

test['destino_heranca'] = (test.groupby('num_familia_raw')['destino_heranca']
                               .transform('first')
                               .replace('','nao informado')
                           )

Output:

  destino_heranca  num_familia_raw
0               A                1
1               A                1
2   nao informado                2
3   nao informado                2
4               C                3
5               C                3