I see why you're hurt, definitely, but I agree with PP that it's natural.
My son is much closer with my mum than my in laws - it upsets me too, and I wish it were different. However, I know it's not my in-laws fault, it's mine really.
I'm much more comfortable with my mum, naturally! I don't care about her seeing me at my worst. So in the first few weeks when I was having a really hard time with feeding and I was a total wreck, it was my mum that came every day. My in-laws stayed away completely because they were being respectful by giving me space.
If my house is an utter tip (which is often!) or I'm still in my PJs at 1pm, I have no problem with my mum coming round. But when the in laws ask, I always put them off for a few hours to get things and/or myself straight. I'm sure this probably makes them feel that they have to give me warning, and that they're an imposition.
In my case, I know it's my in-laws trying to be respectful of me as the primary care giver to their grandchild. It's up to me to try and get closer and more comfortable with them, I think.
I might have missed it, but how often do you ask them over? Maybe SIL offers more invitations?
What are they like as people? For example, my in-laws are very polite and reserved, they would just never want to come over if they weren't totally sure that I'm happy with it.
Hope you figure it out x