>>297
I think it's a run-on sentence and defies faithful translation into proper English. Also, I think "おまけに姉は" is either "おまけに姉の" or possibly "おまけに妹 (or 弟 depending on the sex of the author)は." If I were the author, I'd write:
おまけに私は姉の特ち物を無断で借りて返さないこともありましたし、友達を連れてきて大騒ぎすることもありました。「部屋を模様がえする」と言い出して、趣味の悪い物を部屋に飾ろうとすることもありました。まったく姉にしてみれば「．．．こいつさえいなけりゃ」と何度も思わされたことだろう。
On top of that, sometimes I took my sister's stuff without permission, and occasionally me and my friends whooped it up in our room. I even tried to furnish with vulgar stuff without notice, saying "I feel like making changes in our room." I'm pretty sure she always thought "...if only she wasn't born."