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.