How to not let a rude 14 year old girl bother me?

So i am 19 years old and have a 4 month old baby and am living with my husbands family while hes at air force basic training. and his 14 year old little sister is a rude, spoiled little brat who i have never said or done anything bad to. sometimes ill even do the dishes and put them away for her which is her chore. and she doesnt even appreciate it when i do. and earlier today i saw a notebook sitting on the kitchen table and i opened it and on the first page i realized it was hers cause she wrote about how much she hates her life and thinks everyone hates her and she said something about me that said my brother knocked up a scungy ho bag skank, she is a *****. and after i read that i cried for like an hour i couldnt believe someone could be so mean when ive done nothing to deserve it. so my question is how can i let this and all the other mean things she says and does not bother me? because i still have a few months left here and i dont want to be angry the whole time having to sit at the dinner table across from a very mean person