The book mary or may not be as bad as everyone is stating it is depending on how you look at it, from what point of view you see the book.
I presonally, was sort of in between liking it and- well- not hating it, but simply being dissatisfied. It shed light on a character or two that I'd been wanting to hear about for a while, and that definitely made the read worth it for me.
I believe though, that in the end, last couple of hundred pages or so, Stephenie Meyer was simply trying to cram in too much to make a happy ending with no conflicts or sacrifices, and that in turn, made it hard to read.
There isn't anything quite r-rated in there, perhaps a little suggestive, we all knew that Stephenie Meyer was going to have Bella and Edward get intimate, and if you read the book you will find that it's always the morning-after description, nothing actually being described of the night itself most of the time.
The story itself, however, tends to send the wrong message sometimes, as most reviewers have been stating and I personally agree. Some messages are sent involving how easy choices are in teenagehood, which is very wrong indeed, and other messages that you would hear parents and adults be against, nowadays.
However, you must keep in mind that this is a fantasy book, unrealistic as it is, you have to know that these messages in our world today, technically, don't apply to the book since it has it's own little world. Of course, we can't help but connect our world to that one, so that is how we come to our conclusions.
All in all, I would suggest that you read the book for yourself to conclude on whether it is bad as people are saying it is.
I don't think there would be any problem with your mom reading it either, if she knows you are mature enough as a 13 year old to read the book.
I personally, am 13 years old also, and I found no problem with reading the scenes that most deemed would be a bit inappropriate for people of our age. The book, as a movie rating, would not be above PG-13, I'd think. For me (this little paragraph is my very personal view of the book), Breaking Dawn ends after Book 2. See, the book itself is divided into 3 sections, titled Book 1, Book 2, Book 3, all within the same book, and for me I would be satisfied with Book 2 being the end. However, that is a biased opinion due to my pereference of characters being werewolves. I won't go in depth about that to avoid any more spoilers for you. But yeah, that's my opinion as to the answer of your question.
Myself, I've read th whole series.