Here I am presuming you are teenagers which is how you come across.
As they are step siblings, that would infer there is a parent and step parent present? If so, then talk to your own parent regarding this. Blended families can be extremely fraught with a vast array of issues. It is like any two disparate groups coming together, there is positioning, there is often unsolicited attacks.
People attack for various reasons. Almost always that is because they have their own issues with life. Some because it simply is who they are. There are a great many of the former and very few of the latter. Ergo I would assume she is the former and has her own issues.
Is your step parent the father by any chance? Have you through no fault of your own I wonder, walked into the middle of some daddy/daughter dynamic?
Then I would have to mention your 'very very nice' ..... You do not seem to appreciate that such a persona in the circumstances you seem to define can be incredibly offensive and aggravating
..... The other young lady wants an argument because I suspect you represent something to her that she doesn't much like; living in your home
, in her new life
sans one of her parents ..... She cannot per se attack her parent or your parent ... so you will do. It is nothing personal, just how it is. Life is like that.
One has to suspect she also sees you as a drama queen who gets all the attention when indeed, she is totally screwed up but 'soldiers on' because he innate anger keeps her going.
If you want the best possible chance of decent relevant advice, then I might suggest we know your age, her age, everyone's age, your family situation as per parents and so forth.
You know this information. However without it people can only make educated guesses to try and help you.