Throw away all bottles and ignore your mom's unhappiness.
Tell your girl, whatever beverages now, it's from cup or none. If she throws tantrum, deal with it as how you guys normal deal with tantrums. You can also help her by making it a trip to hunt for a cup that she likes. Thn give her the milk or whatever from that cup.
As for parents, sometimes you really can't please them all the time.. My mom also same, my daughter is her precious.. Sometimes wen we need to do things for my girl's own good but she cries or throw tantrum, my mum will black-face or blame us for making her cry.. Just have to learn to ignore and overlook that, behave as if she (your mom) is not unhappy and live as per normal.. When eventually your daughter settles in, she (your mom) will forget the unhappiness as well..
If she buys more bottle, keep throwing them away until she stop buying..
For me, if I'm pushed to the extreme, i will punish my girl infront of her if I caught her drinking milk from bottle again to send the signal to my mum.. And she will stop giving bottle to avoid my girl being punish by me again..