1. I would like to know whether any procedure to get that receipt/Signature basis on what my Sister has collected the property documents from Magistrate at the time when my uncle transferred the property on my mother’s name as I remember she has collected the original documents from magistrate. If yes how can I get that proof?
First of all there no magistrate who will register the property, it may be the registrar who you have mistaken for magistrate. If she collected the document at the time of registration while accompanying your mother to the registrar office, it may be due to her own influence or by some fraudulent mode. Instead of probing the past fraud you may apply for a certified registration copy and procure a copy by yourself from the registrar's office.
2. What will be the valid ground of eviction from my father’s side so that court doesn’t deny.
It is your father's property, he wants her to leave the property which has been illegally occupied by her. This alone is a sufficient and strong ground to seek to eject her from the property.
3. Can my Sister get stay on eviction due to case is under trial with mother or other reason?
No, she cannot combine that property case with this, she is not having any right in this property so her application for stay my not be considered by court, but your father has to be vigilant about it.