Is it obligatory for the wife observing iddah to spend her iddah in the home of her youngest son where her husband passed away, or in the home of her eldest son where his body was moved after his death?

The default is for a woman to observe her 'iddah (waiting period) in her marital home. However, if there is a necessity or benefit in moving, then it is permissible for her to do so. Her children, both sons and daughters, are obliged to serve her and provide for her if she is in need.