My 2 children love their grandmother, but she only cares for one of them! What should I do?

I am married with 2 children; One which belongs to my husband and the other from my past relationship. Before my youngest child was born, the one that belongs to my husband, my mother in law was crazy about my oldest child. She called him, took him out, bought him gifts and so on. About a year after my youngest was born, I noticed a change. She call to speak to my youngest, but not my oldest. When my husband asked her about this she said my oldest child do not like talking on the phone, which is true, so what is the point of calling him. My husband and I 'swept it under the rug'. Today, I got a call from my mom. My mom told me she spoke with my mother in law and they were talking about grandkids. My mom said, "I do not know what I am going to do with all my grandkids, they are growing up so fast." (my mom has 10 of them). My mother law said, "well I do not have to worry about that seeing that I only have 1.

That hurt me because my oldest son loves her so much and she is not even claiming him anymore. She found pleasure in having a grandchild with my oldest child, but now that she have her own biological grandchild she act as if my oldest child does not exsist. I want your opinion!!! How should I handle this?? We have talked to her in the past about her actions or non actions with my oldest son and she changed for a little while then she go back to her ways. My husband does not like it. He is in the process of adopting my oldest son. SO again, if it was you, how would you react to this situation. Thank you so much for responding!!