How to tell my 13 year old stepdaughter

Hi all, I'm about 8 weeks along this is my first and I have a wonderful 13 year old stepdaughter.  Due to circumstances beyond my control we have her every other weekend and half of summer and split holidays with her mom who lives 4 and a half hours away.  My husband has always made sure she has every opportunity available to her working hard and even driving 18 hours in one weekend to see her for one day (for about a year he did this every other weekend, thankfully now her mom drives one way with my husband giving her gas money).  Her mom is remarried and has 2 little kids so this won't be her first sibling.  My stepdaughter made a comment a few months ago about how she likes the peace and quiet by us like it's a little break for her.  Since we don't get her that often when she is with us 100% of our attention is on her.  We have a great relationship and my husband and her are very close.  I'm worried she won't be happy we are pregnant, I think it would be different if it were a few years ago but now she is over that wanting a sibling since her mom has two other kids and also I suppose due to her age.  I'm wondering if anyone has any experience with this and any suggestions on how to tell her.  We want to make it special but also don't know if she is too old for that.  We will let her know that she will still get alone time with dad etc. and we will still do things when we can.  We plan on waiting until 12 weeks but she might know something is up since I've been so sick.  Any advice?  Thanks, sorry this is so log .