I grew up in the Bronx. I watched the Towers go up, I watched them fall down.
If you can't stand for the 90 fucking seconds it takes to remember those who died in 9/11 and for the soldiers fighting overseas and to sing "God Bless America", which is how we usually take the 7th inning stretch in NYC, then, well yeah, my sister should kick your ass
Because we live where we do, these kids have the right to do whatever they want to with regards to standing for GBA.
In the setting of a privately owned and maintained facility, they also have the right to be removed from the premises.
Your family member does not have the right to kick their ass.