When a first Lord dies his oldest surviving son inherits his title, if he has no sons then the title passes to the husband of his oldest daughter. If the Lord has no daughters the title passes to his oldest brother, if the Lord has no brothers then it passes to his oldest sister. If the Lord has no siblings it passes to his cousans in the same way.
The idea is to keep as close to the direct line as possible.
Hear's the problem. If a Lord's oldest daughter is unmarried then the title in held "In trust" by her oldest paternal uncle or father's male cousan. These "Regents" have considerably reduced power, their main function is to maintain the holdings of the family until their "charge" is married.
Essentially without a First Lord a House is very vulnerable.
To further complicate things; if the daughter of a First Lord marries a First Lord then on the death of her father the two Houses fuse, unless she has more than one son. In that case the second son would inherit her title apon her husbands death. Until that time her husband holds her title "In trust".
If she never marries it passes to the Regent on her death. If everyone dies out then the title returns to the Crown, since there is no crown there has been a steady fall in the number of house in the last 5,000 years.
Still with me? Okay.
How this relates to Drago's problem: Since houses rank themselves by relative power and wealth the big starbust has seriosly shaken up the pecking order.
The result is that house Flax and Relinion's have deadlocked on an equil power basis. With no King to resolve the dispute the Council of Lords is bickering. The big problem is that all Allessa's male relatives got atomised. So she's it. The problem is she's madly in love with Halren Flax (To be fair she hasn't seen him in nine months) and refuses to brake her betrothal and marry someone else, which under the circumstances she can do.
So Drago has to get them wed and have Hal name him Steward and Marshal-General so that he can get things moving. Although he already holds Marshal-General as a title its not technically legal.
The Highest Rank within a House is the First Lord, who is styled:
High Lord: Lord-Leutenant-General
Not all First Lords are military officers. If a Lord proves himself competant he will be awarded the additional land to raise his rank, which will be stripped from the man he replaces.
All other Noble Officers are styled: Knight-(Rank)
There is another General rank, this is Marshal-General, these men are non noble and are invested with their power by a Lord-General and therefore outrank all other officers. When the Lord dies his Marshal-General loses his superiorety but retains the Legions he directly commanded.