Question Summary:
Is it sinful for wife to stay in the same house as her in laws when her husband is not there?

Question Detail:

The husband works abroad. The husband want wife and daughter to live with her family when he is away because he read somewhere if the husband is away for a long peiord of time it's better for the wife to live with her family than in laws (which also include brother in laws and brother in laws are non-mehrams).
But the wife and the husband's mum want them to stay in the husband's mum's house. Wife and mother in law get along well. Mum wants the wife and child to live with her because she want their company. Note that in asian culture it is tradition for wife to stay with the in laws and people gossip if they don't.  
Now, the family are God fearing and there is enough separate space for the wife. The wife is the first cousin of the husband (and also his brothers). The family did know each other well before marriage. And the brother in laws are much younger and have grew up with the brother's wife as she was like a older sister (even though they are award cousins are not mehrams).
Is it okay for the wife and child to live with her in laws (including brother in laws) if there is no fear of harm when husband is not present? Is it sinful for wife to stay in the same house as her in laws when husband is not there? 

Answer :

In the Name of Allah, the Most Gracious, the Most Merciful.
As-salāmu ‘alaykum wa-rahmatullāhi wa-barakātuh.
 
In the enquired situation, the wife may stay with her family or in laws. Whenever she goes to the in laws house, she may observe the laws of Hijab from her brothers in law.
 
And Allah Ta’āla Knows Best
Mufti Ebrahim Desai.