Juice for 15 month old

I’m not like anti juice or anything but I try to limit my babies juice to about 3 ounces a day. My mil and sil always fill up my sons sippy cup with juice when they are over, and I really prefer that he drink water during the day. They don’t listen to me tho and roll their eyes because I am a FTM so they think I am being ridiculous. Should I just let it go, or is too much juice bad?