I came to say two things
One, it is true that sometimes MIL's dont get the same attention that a mother would get. But if you are warm and open to your sons GF and you treat her like one of your own, I think that she will love you as much as she could love her own mom. I have had two very serious bf's, one lasted two years and the other is my OH now. the two year bf still lived with his mom so I got to spend lots of time with her. We would go shoe shopping and she would buy me clothes, we would do girly things together while my ex played video games or whatever he did. I honestly loved and still love her like she was another mother to me. My OH's mother is cold and distant and doesnt really talk to me much, shes also a bit judgemental and I have a very hard time bonding with her. I often mourn the loss of a second family when I think of my ex's mother.
I just wanted you to know that you seem like a very thoughtful person and I'm sure that any of your sons GF's will love you very much!
Also I have no had a child yet but I really, really, want a girl. I joined this board in anticipation of getting pregnant and maybe ending up with a boy, or just to read in advance to get some understanding of why I feel like I would be so disappointed if I had a boy. I've told people before and I always got the whole "Oh come on how could you say that!" so I've just stopped saying it altogether and I'm thankful that there is a place to talk about these things and hopefully not get judged. You're not alone, just remember that.
I hope that your DS2 is easy to care for so that you can be quicker to bond with him, good luck with everything!