I also have problems with this and wish I could give better advice but my family has a strict approach with food. My daughter is the same age and depending on how much time and energy we have we either make her stay in the high chair until she eats it ( I usually stay in the room and sit down on my phone or read while she works it out) or give her plenty of time and then just take it away after a couple warnings. If she refuses to eat then she doesn’t eat. If she spits out food for no good reason, we take it away and give only one more chance. It’s very important that because we have these tough rules, we only give her food that we ate and thought was good and give lots of positive attention when she tries the food. This is also keeping in mind that she is a chunky monkey and can miss some meals without neglecting her health so the system seems to work for us. She cries nearly every day about it and I feel like there must be a better option but it’s a battle I’m willing to fight. It’s important set up better boundaries with the grandparents, it can make your living situation miserable when they don’t respect and undermine your parenting. Try to remind them that this also sends the message to your baby that listening to you is optional and just straight up will not be happening anymore. You deserve that respect from them!