OP, I hear this. I stutter the worst around my mom. Even my bf at the time was amazed how my speech would be really bad when I visited her, then like a switch would improve on the drive home. My mom (well meaning) has pointed out my speech on bad and good days so I think subconsciously I feel like I might be put on the spot at any time. In addition, I think certain people cause us to stutter because of stressful associations to them. Your mom sounds like one of them, she seems to have a big personality.