RANT: My wife's family keeps undermining our parenting

Okay so her parents are constantly telling our kids to ignore our rules when they visit. We said no screens before bed but grandma lets them play on tablets till 10pm. We're trying to teach them to respect authority and follow through on consequences but it's impossible when family is sabotaging us. My wife doesn't want to confront them because it causes drama. This is driving me crazy and I don't know how to handle it without blowing up. How do I lead my household when extended family won't back me up?