I hear you, and I don't always feel mature about it, either!! I have a monkey mind, though, and if I don't try to calm it down with rational thought, negative thoughts will eat away at me.
My in-laws do bug me a lot, because I tend to internalize and feel like they don't love my sons if they don't verbalize their support. Like you, I am always careful to genuinely applaud anything my nieces and nephews do. I truly do think, though, that it probably stinks to hear about another child constantly outshining ours (in benchmark areas like school) when each of our children are the center of our world.
