What is insurable interest?
As per Wikipedia – “A person has an insurable interest in something when loss-of or damage-to that thing would cause the person to suffer a financial loss or other kind of loss.”

Yes, you can insure your brother. Wikipedia –
The insurable interest of family members is assumed to be emotional as well as financial. The law allows insurable interest on the presumption that a personal connection makes the family member more valuable alive than dead. Thus, husbands/wives have an insurable interest in their spouse, and children have an insurable interest in their parents (and vice-versa). Brothers/sisters and grandchildren/grandparents are also assumed to have an insurable interest in the lives of those relatives. But cousins, nieces/nephews, aunts/uncles, stepchildren/stepparents and in-laws cannot buy insurance on the lives of others related by these connections.
Can I pay with cash?
As per above; No, need to pay cash.