He had his own apartment so I doubt his mother even saw him when he got home, especially if he got in, in the early morning hours. I think if he had blood on them, he would have gotten rid of them, plus that is the one thing we do not have. We don't have any DNA anywhere!
His apartment is one of the buildings that comprises the family home. It is only a couple of steps from the main house. She may go in there in the mornings to pick up clothes, etc. Or she may be able to corroborate what time he got home, etc. All speculation, but they are questioning her for some reason. Sigh. Your guess is as good as mine (probably better since I've been gone a couple of days!). :o)