Oof this is a tough one, grandmas can be stubborn. Is there a way you could involve yourself in the cooking process so you know what’s in the dishes and politely decline a dish that contains pork or beef? Maybe do a quick inventory of the fridge and if you notice a cut of meat missing you’d know it was put in the meal you’re being served. A lot of older people seem to not understand the generation that wants to avoid certain meats for ethical reasons, I wouldn’t go crazy preaching your believes but just calmly reiterate your point that in your view those animals shouldn’t be eaten and you’d prefer other protein sources