First, let me say that I'm sorry for your loss. An "early" death stings; even when we know that our loved one is with the Lord. (We become convinced that the world should obey our timetables and too easily forget that the world is on God's timetable.)
It may simply be that you weren't meant to be there. My dad passed away in January. My mom, my sisters, and I spent that last week tag-teaming in his hospital room. The Lord didn't take him home until after my mom had decided to go home for a few hours. We figure that He didn't want us to be there for the end. The same may be true for you. The Lord may not have wanted you to go through that last moment; especially when she was already in His care.
In short, don't beat yourself up. If the Lord wanted you there, you'd have been there.
I forgot to mention that my oldest sister didn't make it out in time; however, she had been in denial about dad's condition all throughout his decline. She simply wasn't ready for him to go. The rest of us are all convinced that God didn't want her to be there. It was easier on her to have it all over and done with by the time she got there.
Trust in his wisdom.