Is this strange for showing my nieces(the older one is ten and the youngest one is six) Smile Hd and the youngest one actually loved it while the oldest one felt sick? What should I do?
You should not show children things that are inappropriate for their age, ever. You are not their parent, and you are not equipped to decide what they are ‘ready’ to see. Excessive blood and gore is not something children are equipped to handle, and you made a massive lapse in judgment by showing it to them.
While I would personally recommend informing your brother/sister what you have done so that they could keep the children away from their obviously-irresponsible and damaging relative, for now I will try to temper my wrath and simply suggest that you do not ever do this again in the future. Stick to showing them videos of very safe, age-appropriate material. There are lots of videos that are safe for children to watch, and you should have no problem finding many cute kitten videos and the like for them to see.
In short, you should know better. Perhaps in the future you should keep a responsible adult around while you have your nieces with you, to be certain that they are being chaperoned appropriately.