Maintaining a daily bedside vigil since Bobbi Kristina, 21, was hospitalized on Saturday after being found unconscious in a bathtub, the R&B singer "stands over her and touches her hair and calls her 'baby girl,' " a Brown relative tells PEOPLE in this week's issue.
Along with other close family members gathered in the intensive care unit, Brown has been singing to Bobbi Kristina – hymns and gospel songs as well as songs by her late mother, Whitney Houston. (Bobbi Kristina has since been moved from her Roswell, Georgia, hospital to Emory University Hospital.)
"We'll sing 'Jesus Loves Me,' 'Amazing Grace,' but also some of Whitney's songs," says the relative. "We've been singing 'Greatest Love of All.' Bobby has sung that to her too."
Although his daughter remains unresponsive and can breathe only with the aid of a ventilator, Brown – who is making all the decisions regarding her care, says the relative – is not ready to give up hope.
"Bobby has told us, 'If American thinks she is dead, they will stop praying,' " says the family member. "And at this point, we want people to pray for a miracle."
Over the years, "Krissi" and her father, who turns 46 on Thursday, "have had their ups and downs," a source told PEOPLE after Houston's death in 2012. "They get into big fights and then they don't talk for a while. Sometimes the fights last for a day, sometimes longer. They're both pretty stubborn."
A different Brown relative agrees that the relationship has been a rocky one. "They'll go for months and months without saying a word to each other," says the relative. "They've very alike, which I think is the issue. They will be talking and loving to each other, and then a little thing will turn into something big and they won't talk for a long time. They know how to push each other's buttons."
But through everything, says the relative, "Deep down, they love each other very much. It would kill Bobby to lose her. I mean literally kill him."
Reporting by STEVE HELLING
For more on Bobbi Kristina, pick up the new issue of PEOPLE, on newsstands Friday
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