mom upset mother-in-law can't make the baby shower

My mom and sister are throwing a baby shower for my husband and I. It happens to overlap the weekend our niece is having a cheerleading national competition out of state that turns out my mother in law has already been planning on going to the competition for a couple months now and won't be able to make the baby shower. This has apparently has really upset my mom who says "there will be other competitions and should attend the baby shower instead because it's her sons first child and all." I personally understand that plans have been made way before my mom planned the shower and it would break my nieces heart if her grandmother wasn't there for her first competition. I said it wasn't a big deal, but my mom won't let it go and can't understand why we're not as upset about it as her about it.  Any advice on how to deal with this situation?