Can nursing while pregnant take nutrients from developing baby?

Ok my sister in law’s threw this at me today... I’m still nursing my son. He’s 13 months old and it’s only at night and fairly infrequent. I’m 7 weeks along with my third. Well my nurse last week said I should stop nursing and when I asked if either of them had heard that they said yea because the nutrients that are supposed to go to the new baby are going to the breast milk.... how is there women who tandem nurse then? I feel like they are just uninformed but of course the hormones have me questioning my own instincts and worrying. Anyone hear this before?