Is it legal for a 12 year old to stay home with a 5 year old sibling and no adult supervision for 8-10 hours a day while the parents work?

It depends on what State you live. In Canada 12 years of age is legal to look after children younger than yourself. However 8 - 10 hour days is a big weight on a 12 year old and you should think of getting an adult in to look after your children so your 12 year old can live the life of a 12 year oldinstead of being a built-in babysitter. If anything happens with the 12 year old or the young child(ren) then Child Welfare (that's what we call it in Canada) can investigate.