Volunteer work

[StolenBase]

My wife and I are building a house in US. Is it legal for her brother and his friend, both citizens of a VWP country, to enter on ESTA for the purposes of helping us with the house without any compensation?

[daggit]

From the State Dept website: The following are examples of activities not permitted on the VWP and require visas for travel to the United States:
study, for credit
employment
work as foreign press, radio, film, journalists, or other information media
permanent residence in the United States


Your friends are in a grey area, possibly encroaching on 'employment' as purpose of travel. Volunteering is considered working without pay, with very limited exceptions.

[VNK]

My wife and I are building a house in US. Is it legal for her brother and his friend, both citizens of a VWP country, to enter on ESTA for the purposes of helping us with the house without any compensation?

No. That's consider working. It doesn't matter if they get pay or not.

This is just like a grandmother coming to babysit her grandchild. The US has refused visitor visas for these grandmothers. You can visit your grandchild, but you can not babysit.

If you would otherwise pay a person to do the work, then it's illegal for a VWP participant to do the work.

VWP are here to spend money on a vacation. They are not here to do free work.