Who is this person and what has she done with my child???

First, let me say I am not one of those moms who thinks her kids are perfect.  Well, the baby is, but she's a baby so what could be imperfect about her?  The other two though, I know they are not perfect.  I know Sabrina is a little too sassy sometimes, and I know Corrine likes to assert her 4 year old self even though she is only 2 and she gets away with a little bit of it because she is so darn cute about it.  I know all of this.  However, our weekend away this past weekend left me slack-jawed asking my husband, "What the hell happened to our child this weekend?"  Sabrina was atrocious even by Sabrina standards!  We went to the Poconos with my in-laws.  My father-in-law likes to try to plan at least one family outing up there for as much of the family that can make it.  Now that there are a bunch of little kids in the family again, he likes to bring us all to Knoebels, an amusement park up there somewhere.  It's fun, and this year my niece, Alexandra (Sabrina's very best favorite cousin) and her mom were with us as well as my sister-in-law and her family which includes one of Sabrina's other favorite cousins, Robby.  She was so excited to see them this weekend.  This is exactly the reason why her behavior shocked me so much. 

My niece is 2 months older than Sabrina and there is a lot of innate competition between these two girls.  Their fathers are brothers and watching these two girls interact is like watching my husband and his brother interact.  Sometimes this is good, and sometimes is volatile.  But I can't even say that my niece was the reason for her behavior, because all of the horribleness was directed at me and my husband.  Everything, and I mean EVERYTHING was an argument.  And not just your everyday, run of the mill 4 year old stubbornness.  She was literally spewing rejections, tantrums, and fresh back-talk at every turn.  I was really left stumped as to how to deal with her.  I have no problem disciplining her in front of our family, but nothing I did could snap her out of it.  She did something at one point, I caught my mother-in-law's eyes bugging in that, "OMG, I can't believe this kid just did that" look.

I just could not figure her out for the life of me this weekend.  Was she this way because she was looking for attention?  Jealous of her cousin and/or sisters for some reason?  Wanted to show everybody she was in charge?  Or do they just think that acting out will get them what they want at this age and there is no need to analyze it to the nth degree?