The twist is that the parent in question is my MIL!
MIL and I will go shopping or do lunch. Then, the next time we are all together with SIL and BIL, if MIL mentions that her and I went out, SIL ( and even BIL for that matter) will say in a snotty tone, "Oh, it Must be Nice to be able to go to lunch." So, now MIL won't even mention it anymore since that is the general reaction.
So, if the subject ever happens to come up, what is a good reply?