Rice cereal? In laws

My in laws are insisting that I start giving my baby girl rice . I am Only BF and she just started to get assisted sitters foods stage one . My pediatrician said she doesn't need any of it she gaining slow and a bit on the petite side but that's just her he's not concerned. She is almost five months and 12 pounds . My in laws don't really believe in BF they don't think it's enough for the baby! Now I dunno what to do any advice ?