Although Nangong Nuannuan wasn't the Chi family's granddaughter-in-law yet, she truly loved Chi Yang and the Chi family. She treated the Chi family as her own family. Old master Chi even felt that she treated the Chi family better than she treated the Nangong family.
If anything really happened to Chi Yang, he would definitely not let this girl who was not his granddaughter-in-law but closer to him than his granddaughter-in-law suffer.
He would become her right arm, her backing.
In this life, if Nuannuan were to stay with the Chi family, he would pass the entire family to her.
If Nuannuan were to get married one day, he would treat her like his only biological granddaughter and marry her off in a Grand and glorious manner.
Although he knew that his grandson would be sad in the underworld, he also knew that Chi Yang would only be at ease in the underworld if he could make Nuan Nuan happy.