Move over, Brady Bunch! Ben Affleck is finalizing his holiday plans, and they include not only his girlfriend Jennifer Lopez and her twins Max and Emme, 13 — but also his ex-wife, Jen Garner, 49, their children Violet, 16, Seraphina, 12, and Samuel, 9, Garner’s current beau John Miller and Miller’s two kids! “Everyone is nervous and hoping things go well,” a source tells Us of the plans. “But Ben and Jen are getting along great, so they have high hopes that it will be fine.” The source notes the reason Affleck, 49, is so keen for the families to get along is that he’s gearing up to propose to Lopez, 52. “They’re going to be engaged very soon,” the source confirms. “They want to spend quality time together so everyone can…