Equality between Children

Q: Please advise on the following:

A father has three children. A daughter and two sons. One son is married.

The father owns a business and both the sons work for the father, while the daughter helps out occasionally.

For the son that is married, the father gave them a house, pays for their water, lights, car payments, fuel, phone payments, etc. The father also pays for numerous holidays and gives them spending money for the holidays. The son is then given a large salary at the end of every month. The father in law is also required to pay for all the daughter in laws wants.

The other son and daughter both live with the parents still and thus the basic food and housing needs are taken care of. The unmarried children are given an allowance which they use for any extras such as clothing, car payments, holidays, hobby's, etc. This often leads to a lot of conflict in the family.

My question are as follows:

1. In a case like this, is the father being fair and just to his children?

2. Is there any difference in providing gifts, or support to the married son and unmarried son/daughter?

3. Is it compulsory for the daughter to work in the business to be given the same as the son while the parents are alive?

4. If the father spends on his wife (mother), is it compulsory for the daughter in law to receive the same?

5. Is it compulsory for the husband to provide the wife with an allowance? (not to be used for any households expenses or personal needs)

6. What obligations do the parents have towards the daughter in law with regards to financial support for needs and luxuries?

7. Is it the parents responsibility to buy and pay for the married son's house, car for the daughter in law, supporting the children on the grounds that he is working in the business and is entitled to it? (this excluding a monthly salary)

Q: I know a man in our neighbour who is married twice. He divorced his first wife and has a daughter from her who is living with him not with her mother. And from the second wife he has a daughter too. But he does clear injustice between his daughters in many ways.

On social media he always posts pictures of his daughter from his current wife and never mentions the elder daughter. I know that girl is hurt by the deeds of her own father. And instead of asking her father, people ask that girl why your father does not post pics or mention you, why does he not care about you? She is always bullied by the people. What do you say about that man and will the poor girl be rewarded in the Aakhirah due to her sabar? 

Q: I want to know that if a child was expected to do certain things (provide,work etc) for the family by a certain age even when having other responsibilities isn't it only fair to expect all the other children, when they reach that age to be expected to do the same (especially since they have lesser responsibility than the first child did) as in Islam all the children should be treated equally, so just like they will have give driving lessons to the others as they did for the first child the same way the other children should be expected to do the same chores and work for the family the way the first child did.

Q: Is it gunaah if I gave my property to 2 of my children an not the other 2?