You can use /ignore in game to ignore players that are being rude to you and your sister. This might help.
Otherwise staff is always here to help you or talk to. If it's happening and a staff member is online make sure to /msg them so they can properly assist you and defuse the situation.
Lastly, I hope your sister can still enjoy this server and ignore the haters. Just make sure to focus on the good, (your friends, the fun times you have and the memories you are making) instead of the bad. This server has such a big positive side, however sometimes 1 player can just ruin it for everyone, but only if you let that player ruin it for you!
That was my worst Inspirational Speach ever, Enjoy and also Enjoy the server <3