First off, let me just say that my mom just passed a few days ago, so my answer is coming from recent happenings...
1. Be glad your mom is around and involved. Enjoy your time with her.
2. I have seen this with my own mother in-law. She enjoys girls more and the older grandchildren more. It is what it is, and she is not going to change (we have discussed this with her already and she sees no problem). So, therefore, we have the choice of accepting it (and her decisions) or not having our son ever see her again.
3. You are not going to change your mom.
4. There is a HUGE difference between a 4 year old girl and a 2 year old boy. My mother in-law only wanted to be with our son AFTER he was potty trained. She said she would not change diapers. And even now, she is not that thrilled with being with a boy (my son is now 9). Her favorite grandchildren are the two girls. She enjoys baking and cooking and going to Tea Rooms and such and, of course, does all of those things with the two girls.
I truly feel for you as I am the only one in my husband's family that doesn't have a daughter. So we get left out quite a bit. It is hard to see the other moms and their daughters out with my mother in-law at Tea Rooms and shopping days (of couse, they show me all the pictures and such). But, it is what it is and I am not going to change my mother in-law.
Not sure if any of this helped.