Surviving Twin Issues

I wrote recently that my daughter, Mia has had a lot of questions about her twin brother, Parker and about death in general.  While I try to be honest with her...I'm just not sure where her questions are coming from.
Last night, she screamed.  She did not want to go to bed.  When I asked her what was wrong, she said, "I miss Parker".  I don't know if she is using that for attention (as she knows she'll get it) or if some how...some way....she really truly misses him and is grieving.  If that is the case, how do I help her?  I feel like a terrible mother today.  I comforted her a while after she said it, but then she just kept whining about everything.  I ended up losing it and yelling and finally, Chris took over.  Today, I'm looking back thinking....was she really sad about her brother?  Did I seriously just yell at her when she is grieving? On the flip side...he was still born and she's almost 5...she can't use this "excuse" for everything, forever.   Then again...is it an excuse? 
Any surviving twins out there????  I need help...