Look at this way, look at the meaning of hostile. Means combative, war like.
What was your sister doing, being combative, picking a fight?
She is most likely projecting what she is doing to you, so doesn't think what she does is wrong.
I think why they are acting like thät towards you, is that sometimes in relationships some couples do that to bond. In some ways they are trying to bring you out of your shell.
Like this movie where this woman has a similar attitude to they claim you are (hostile and another word the woman shown there uses is tempestuous), but théy bond over a game of paintball which is being combative -
It is based off William Shakespeare's work (Taming of the Shrew). In that play the sister has a sister like your sister. From what I understand, it is about two sisters of a father, based in Italy.
In addition, sometimes just have to laugh it off (not take life seriously, more so with family that know you well, not so much with strangers that don't know you well enough), give it back.
Try it and see what happens (sister bonding).