Question

What religion was siddhartha born to?

Answer 1

Siddhartha Gautama/Siddhartha Shakya was a Hinduprince, born and raised in what is present-day Nepal. 
He belongs to Hindu religion ...

Answer 2

Siddhartha's caste was the Kshatriya caste (the warrior rulers caste). He belonged to the Sahkya clan and was born in the Gautama family. Because of this, he became to be known as Shakyamuni “sage of the Shakya clan”, which is the most common name used in the Mahayanaliterature to refer to the Buddha
hope it helps.. :-) :-)