Evil children

So, today sucked. Really sucked. I am really struggling with my kids these days and today was a killer day. We seem to have this HUGE problem. My kids gang up on my niece. She's two - only 8 months younger than Baby Boy. Whenever she comes over, my kids morph into these two mean and spiteful children. They won't share. They run around and try to lock my niece out of their room, not including her in what they're doing. It drives both me and my sister mental. It makes me almost sick to my stomach. Why are they like this? What have I done to teach them how to be so awful?

I know part of it is my own insecurities. I had a rough go in junior high and still hold the fear of not being accepted. It is my highest priority to try and teach both of my children to be kind and inclusive. They're pretty good with other kids. Baby Girl has had some issues with letting people in her room before, but that seems to have passed. EXCEPT when it comes to my niece.

So today was awful. We were having a great morning. Nice, quiet, nowhere to go. Baby Girl and Baby Boy were playing great together...for what seemed like hours. Then my niece arrived. I tried to engage them all. A new tactic, instead of letting them play without direct adult contact. I thought it would help to get our day off to a good start. We settled down at the coffee table to do our letters to Santa. And from out of nowhere, Baby Girl spiralled into a hysterical tantrum that lasted for at least 45 minutes. I couldn't figure out the problem. All she did was scream and cry and refuse to let me leave her. When I finally had her settled down, Baby Boy got set off with something...a sharing issue and then he spent 30 minutes in his room screaming at me. Every time I went in to try and console him and see if he was ready to be nice, he threw another fit and tried to hit me.

After lunch, we put my niece downstairs for her afternoon nap and suddenly the two happy children returned. I DON'T GET IT. The words I hate my children ran through my brain. Where has this behaviour come from? What am I doing wrong? I'm starting to think I should find someone else to take care of my kids when my husband and I go to Mexico in January, because I don't want to inflict this torture on my sister for 7 days....especially considering she's about to have a baby any day.

God, please give me strength and patience and some kind of wisdom.