There is no relation between you and your grandmother's cousin-in-law's grandson.
You are not related as you do not share a common ancestor.
Your aunt's grandson is your first cousin, once removed. Whether or not you can legally marry him depends on the laws of your country, province, or state.
Your aunt-in-law's parents would be your grandparent-in-laws.
You may not marry your cousin. However, there are no laws against marrying your father's cousin.
Your boyfriend's cousin's girlfriend, I reckon.
Her 2nd cousin twice removed.
There are no laws to prevent marrying a first cousin once removed.
There are no restrictions or laws that would make it a problem to date, or even marry, a third cousin.
How the estate is handled depends on the laws of the jurisdiction in which it is probated.
The estate of the grandmother should be closed with the proper procedures and the assets put into the estate of the son. The son's estate will be distributed according to the laws of intestacy for the appropriate state.
Yes, you are related to your brother-in-law's grandson. Your brother-in-law's grandson would be your nephew once removed. This means that you are not directly related by blood, but you share a family connection through your brother-in-law's family.
Your son-in-law is the husband of your daughter. His son is your daughter's son and therefore your grandson, and you are his grandparent.If your son-in-law's son is from a previous marriage, the son is not physically related to you. He is your daughter's step-son, he is your step-grandson. However, you may want to treat him like a grandson anyway, especially if he came into your life at an early age.