It was right for Mustafa Kemal Atatürk's sister to insist on a religious funeral despite his beliefs. This is because family members are the ones left behind to grieve their loved ones. Although Ataturk was responsible for the Armenian Genocide, his family still loved him and they should be ultimately given the rights to have a funeral any way they wish.
Mustafa Kemal Atatürk was the founder and very first President of Turkey, a historical figure and a point of inspiration for the people of his nation. While his funeral may have gone against his personal beliefs, it wasn't an event held for him. It was a ceremony designed to let the people mourn. It was appropriate.
I believe is was right for Mustafa Kemal Ataturk's sister to insist on a religious funeral despite his beliefs. While some have labeled Mustafa as an atheist, I believe this is far from the truth. I believe Mustafa was aware of the problems religion created in a society and he had problems with religion because of this. I do not think it was an indication that he was an atheist, he just didn't like the way it affected people or made them act.
The family deciding to have a religious funeral for Mustafa's death was their decision to make. The individual is dead anyway so providing a religious service on his behalf does not effect the outcome one way or the other. That is just their way of showing proper appreciation for an individual and what was accomplished.
If a loved one asks for something to happen to them after they pass away, you fulfill their last wish. It is wrong to be selfish and do to their body whatever you think is necessary. It is especially disgusting to impart religious beliefs upon a person's corpse after they have died, especially if it goes against the beliefs they had while living.