Deadly Sister...

She sounds like she needs therapy. You guys don’t constantly pick on her, right? Because it’s incredibly difficult to be picked on and picked AT when you’re the little one. The pressure to not irritate you guys, or set you off may have gotten to her. She probably has picked up that ‘adorable’ little knife weilding trick from tv, and as it has gotten results, feels it’s a great way to get you guys to back off. Kids have breaking points- and at ten she’s of that age where you start to feel you deserve more respect and independence than you actually do. (not that she doesn’t deserve respect at all…)

Try talking to her calmly. Invite her up to your room for a sit down sister to sister chat and ask her why she’s so angry. Ask her why she thinks it’s ok to threaten your life with a knife. Let her know that it’s not. Tell her that you plan on going to your parents, or even higher up, if she doesn’t stop.

I’m going under the assumption that she’s acting out (out of desperation and a lack of knowing what her other options are), but it may not be the case. She may legitimately have some mental issues, that she needs worked out.

Bring this up to your parents if you haven’t already.