How do I ignore what my family tells me?

How do I not let my families mean words effect me? My grandmas sisters grand daughter is living with us and she has for a while but cince i had my first child she has been very mean to me and I dont understand why? I have talked to her about it and she says she dosent want to talk about it. I would be okay with that if she didnt try to insult me or my husband every chance she gets. She says things like I have gained weight or my slippers are too noisy or i shouldnt be carrying so much i might drop it all. She is very negative and im just telling myself a million positive things over and over again and its not helping.