Help I would like to know if it's true

Adriana
Today I went to finally visit my cousin which she had twins!! We were both talking and all of a sudden I see her give her kids vitamin d and starts to breastfeed! So I asked y does she give them vitamin d?? She answer back that her baby's pedestrian said whenever she's going to breastfeed her babies she has to give them because breast milk doesn't produce any vitamin d! Is this true?? I have two kids I breastfeed till they were 2yrs old and I just had a baby 5 months ago and never have I heard this