[Lace]

I'm posting concerning my mother's Yorkie, Lace. Lace is 1 year 3 months old, unspayed, and under 5lbs. Lace is a nice dog most of the time, but there is a problem when people leave the house...she attacks ankles and barks viciously. She's been doing this for at least 3 months, and she only goes into a rage when people are LEAVING, not arriving. She has bitten pant legs, but has not hurt anyone yet. I have scolded her sternly, but not angrily, when she has done this to me, and as a result she seldom comes after me anymore. However, she still does this to other people. If I'm around and company is leaving, I'll pick her up and remove her from the room so she doesn't go nuts around the guests. I'm guessing other members of my family aren't being assertive/confident enough? I've mentioned this to my mother. I'm still the only person she (usually) doesn't attack. I'm wondering if there are any training tips for this problem, as I want to get it under control before Lace hurts someone. Thanks for any advice!