Is it permissible for a woman observing 'iddah (waiting period) to travel to spend Eid at the home of her deceased husband's father, provided she adheres to all conditions of the 'iddah?

Your mother must remain in the house where her husband passed away until her iddah (waiting period) concludes. It is not permissible for her to travel anywhere before the iddah expires. This is affirmed by what is stated in the Fiqh Encyclopedia and is indicated by the hadith of Furayah bint Malik, whom the Prophet (peace and blessings be upon him) commanded to stay in her house after her husband's death.