Britney was no where to be seen of the night of Grandes murder. Mrs spears as we know is held hostage by her dad. She may of been forced to kill Grande or find her home.
Britney is a much odder person on this case , she hasn't done anything in year as her dad is her manager and she refuses to make anything under his name for so many reasons as she said.
Her dad has a lot of power over her , he could of easily made her kill ariana and rob her home but it looks like her room hasn't been touched by anyone ever since her death.
Ariana's body was left with some hair but it was patchy and the DNA they have tested has matched loads of people including Britney .