If someone unknowingly bought stolen cement multiple times, and if the seller is revealed, will my husband bear any sin?
The sin of the theft falls on the one who stole. However, the person who used the cement must pay its value to the rightful owner from whom it was stolen, and then seek reimbursement from the seller. If the thief compensates the rightful owner, there is no liability on the person who used the cement. And Allah Almighty Knows Best.
I`m a mother of three orphans. Is it permissible for me to spend on myself from their salary, such as buying a Jilbab*?
All perfect praise be to Allah the Lord of the Worlds. May His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
It is permissible to take a reasonable amount that is equal to your services to them. However, it is better that you don`t. The evidence on this is that Almighty Allah Says (What means): "Make trial of orphans until they reach the age of marriage; if then ye find sound judgment in them, release their property to them; but consume it not wastefully, nor in haste against their growing up. If the guardian is well-off, Let him claim no remuneration, but if he is poor, let him have for himself what is just and reasonable. When ye release their property to them, take witnesses in their presence: But all-sufficient is God in taking account." [An-Nisa`/6]. And Allah The Almighty Knows Best.
* A full-length outer garment, traditionally covering the head and hands, worn in public by some Muslim women.
Is it permissible for a wife to leave her house without obtaining the permission of her husband, and not to come back unless he divorces her co-wife?
It is forbidden for the wife to leave without obtaining her husband`s permission, and it is impermissible for her to ask him to divorce her co-wife as this inflicts harm on the latter and such an act is forbidden in Islamic law.
Is permissible to buy/sell with non-Muslim?
Yes buying/selling for a Muslim with non-Muslim is permissible as regards what is lawful.