When a son of a noble inherits a lord title, what would be the position of his (younger) brother? Is he important? How much can he inherit after their father? Is he dependent on the oldest brother?
One might think this is an easy thing to answer, but titles are a science in and of themselves.
If we are talking about Great Britain, which I will assume we are, then the younger brother would not be a so called “peer” but a “commoner”. Peerage goes from father to son, if there is a son to inherit the title. (Some peerage is created “for life” and not hereditary and in the case of no son you would have to look at the direct linage, but let’s not go there). Below are the courtesy titles. These are possible titles before the oldest son inherits his peerage title from his father.
The eldest son of a Duke can adopt the title of
Marquess, the younger son/s would be Lord (first name, last name).
The eldest son of a Marquess can adopt the title of Earl, the younger son/s would be Lord (first name, last name).
The eldest son of a Earl can adopt the title of Viscount, the younger son/s would be The Honourable (first name, last name).
For sons of Viscounts and Barons, all their sons would be The Honourable (first name, last name).
The oldest son inherits the title. A younger son does not, instead he would have a courtesy titles (as long as nothing were to happen to the oldest son).
So, in short, no; a younger son would not be important when it comes to peerage but he would have some form of title as seen above. Your character, the younger son, would not have a special position beyond the fact that he belongs to the aristocrat. He will not have a seat in the House of Lords, for example. (But he is allowed one in the House of Commoners). If your story takes place in the Middle Ages, he might pursue knighthood. If it takes place in the 1700 or 1800s, he might try to work towards a career as perhaps a lawyer or a politician.
As for what the younger son inherits, it’s the older son who inherits the estate and the land. Few countries had such a strict primogeniture as Britain and while other countries would divide inheritance between at least the sons, in Britain all of it went to the oldest son. However, since the twelfth and thirteenth centuries landowners made use of the maritagium - which meant that the landowner could donate land to daughters but also younger sons once they married. In the fourteenth century the maritagium became an entail and the landowner could more freely decide what was to happen to his land after his death.  How much “money”, to make it simple, the rest of the children would inherit would differ. It depended on the settlement which would be made on a sliding scale and take into account numer of children and the money in total as well as what is to be inherited to the oldest.  So, once you decide how much the family is worth, you can decide how much your characters will inherit.
Coldspot Issue 2 is half way through being made. Covers need to be printed and pages folded and bound. Will be up at the weekend on bigcartel or something. Sorry for the delay, starting back college and finishing work took up alot of time.
For those involved I will be mailing your copies out or meeting up with you in college.