If you are the property owner, you can do all of the building and mechanical work.  The plumbing code requires you to be currently living in the property in order for you to do the plumbing work – your brother-in-law can certainly HELP you, but unless he is living with you he should not be doing the work without you.  The electrical code requires that you are either living in the home or will be living in the home once the work is complete.  Your brother-in-law should not be helping you unless he is a licensed electrician; however, your mom or dad can help you – no one else.

Here’s another way to look at this answer:

Residential and you are living in the house now: You can do all of the building, plumbing, mechanical and electrical work.  Please note, the electric code allows for YOU to do the work yourself or with the assistance of your parent (friends, neighbors, or siblings cannot help you).
Residential new home and you WILL live in the house: You can do all of the building, mechanical, and electrical (same rules as above).  You will need to hire a licensed plumbing contractor to do the plumbing work.
Residential and you rent the house to others (EVEN IF they are living there rent-free): You can do the building and mechanical work.  You will need to hire a licensed plumbing contractor and licensed electrician to do the plumbing and electrical work.
Commercial properties: You can do the building and mechanical work.  You will need to hire a licensed plumbing contractor and licensed electrician to do plumbing and electrical work.