Things between Blake Lively and Ryan Reynolds are getting so serious, they spent the holiday weekend together along with Lively’s family. On Saturday night, the couple went to see “Mary Poppins” on Broadway with several family members and afterward went for late-night treats at Serendipity 3 on the Upper East Side. We’re told Lively and Reynolds, both wearing hats, held hands while they ordered a round of desserts, and remained largely unnoticed by other diners. Sources told us two girls sitting at the next table noticed Lively’s phone, saying: “That girl has the same iPhone cover as Serena on ‘Gossip Girl!’ ” — not recognizing the actress until her party got up to leave.