Sounds like it's tough being a step mom.  I would really try to work on your relationship with your step daughter because as she gets older, it won't get any easier.  Maybe it would help if you just had some time alone with her to help build your relationship.  If you take Dad out of the equation, she may be more likely to have fun and less dramatic.

Umm... I don't want to sound condescending here, but she's an 8 year old child. What 8 year old child doesn't get under the skin of the adults around them? I know mine certainly does!

You are the adult in this equation. Rather than try to change the personality of a child, you might want to consider your own personality and see if a little bit more tolerance can be slotted in there somewhere?

Be aware that your step-daughter might also have an issue with regards to you being the "replacement" mother. That is something that isn't a personality thing but a rebellion thing. Might be worth checking in with a councellor about that if you thing that's part of the problem.

And I back up what Arna said. If you don't get along with someone's personality, it's most likely because you are too much like them or there's a jealousy issue to address. As the adult, it's up to you to sort out your issues to get along with this child. As for your sister, that's something that both of you need to work out together.

I found I couldn't get on with my step dad very well.  Why?  Didn't know at the time, but now I do.  We are very similarly minded, don't back down and have short temper fuses.

Often the personalities we clash with are very similar to our own.  We also tend to not get along with people who show qualities we either don't like in ourselves or wish we had.  Sounds crazy, but it is actually true.

If everyone got along all the time and agreed on everything, our world would be a very boring place.  Working to understand the people who annoy us also brings self understanding and awareness.

Take the time to sit down and write out what is preventing you from getting on with your sister and step daughter.  I think you would be surprised at what you find.  It might also make you feel better about it too and give you some idea of where you can improve on things to make it a little better.