Nobody DEMANDED the sign at Fatima. It was freely given by God.
Therefore, your objection to it is misdirected and pointless.
What sign? There’s no clear account of what the “Fatima miracle” even is.
This is literally the MOST confusing “miracle” ever.
I have read a few books about Fatima recently and, actually, Lucia did ask the Virgin Mary for a miracle. Not because she or her companions had any doubt at the time (However, between June 13th and July 13th Lucia had some doubts because her mother tried everything to make Lucia say she had lied. The parish priest said the apparitions might be a work of the devil. Lucia had a dream in which the devil tried to drag her down to hell. In other words, she was in a very difficult situation. When July 13th came, all her doubts disappeared.)
Lucia wanted other people to believe the apparitions were real.
On July 13th Lucia said to the Virgin Mary: “I would like to ask you to tell us who you are, and to work a miracle so that everybody will believe that you are appearing to us.” The Virgin Mary answered: “Continue to come here every month. In October, I will tell you who I am and what I want, and I will perform a miracle for all to see and believe.” The above quotations are from Lucia’s Memoirs.