Well, the wonderful thing about being a human is that we don't have to like everyone. But as to how you behave towards her, that may merit some changing. Begin by putting yourself in her shoes. Begin to look at her with eyes of empathy and understanding. A mother spends years raising a child.
Some mothers invest their entire being into their child.
Their child is their source of identity, purpose, security and meaning. They may lack in other areas of their life, and their connection and relationship to their child is their only real source of joy and meaning. On some subconscious level, she fears that you will replace her and ultimately steal her son's love. In many respects, her fears are valid. Many sons, and sometimes daughters, abandon their original families once they have securely established themselves in a new family that fits their needs.