S.B.
I don't think it's strange at all. I adore my SIL. We are very close. But if I could plan a girls weekend away with my mom and sisters, I'd do it in a heartbeat and not think of inviting my SIL. It's not a slight to her. It's about my mother getting some uninterrupted one on one time with her girls.
And my mother in law does plan special things with my SIL where I am not invited. I have never given it a second thought. I've been married for 16 years. My MIL and I have a great relationship. I know she loves me and vice versa. But I cannot compare to her daughter. The relationship is completely different and separate from me. I think it's great when they get together. Even when they go someplace fantastic that I envy, I am glad they get to go.