mom vs mom-in-law

My mom is a difficult person - vindictive, manipulative (very fond of money), and hysterical - no one likes her. I have always hated her because she made life hell for my dad (who's a wonderful person) before she finally left him. I do not live with her but I know how to handle her. I want not to be attached to her - I just want take care of her (financially) and I'm there when she needs me. Though I "hate" her, I'm still "connected" to her and there are times when I simply miss "my mummy", her hug, her coffee….
The problem now - is that I'm unable to reciprocate the love that my mom-in-law showers on me. She's a wonderful person - sensible, generous, very loving and treats me like a daughter. She does everything that my mom should be doing now for me. Yet I can't connect to her like I do with my mom - like I when I had a fever recently, and she was looking after me so well…I was wishing my mom was there. I sometimes get unnecessarily irritated with mom-in-law yet I take so much crap from my mom. Why? It's because (I guess) I have childhood memories and a bond with my mom. No one has noticed all this, however, I hate myself for giving a wonderful person (my mom-in-law) less value than a horrible person (my mom). Does this happen to everyone? Is a biological bond so important ? Should I make a renewed effort to really love my mom-in-law? Or should I let things be?