Informal letter

I'm taking an exam in a few days,my writing skills will be tested and the indications are very clear: I have to write a formal or informal letter to a friend apologising for not being able to attend a party I had already said I was going to come. Can a ptofessor tell me how to improve what I did?

Hi Jill,

I hope you're doin well. I'm writing you this letter regarding the party at your house next Saturday.

First of all, I need to apologize to you because I won't be able to come to the party at last. My parents are going on a business trip,and they had nobody to take care of my little sister. As you already know,my parents have always supported me in everything,so when my mother asked me if I could stay home and babysit my sister,I was caught between a rock and a hard place, I had no choice,I felt a moral obligation to help her out with that. Yes,I know, we have been waiting for this party all year...I'm really sorry that I'm letting you and our school friends down,but I just couldn't do that to my parents. Besides, the summer camp I've been dreaming of since I was a little kid is just around the corner and despite the fact that my parents would let me go there whether I took care of my sister or not, I prefer to show them that they can count on me whenever they need me.

Although I won't come to the party,I'll send you the present I'd bought for you, if you don't mind. Please tell Jane and Mary that I'm really sorry that I'won't be there and that I'll find the way to compensate them for that.

Take care!