Marriages And Adoption
Marriage is really depending on your marriage contracts. Your own family members will however always stay with your family. Children also need to be negotiated. I first thought abouta an elaborate rule system ala Imperium Nova but then decided against it...its unecessarily complicated.
getting children is actually quite simple: It will be randomly rolled in the 5 year passing periods. on a 2,3 no child is born. on a 4,5 a child is born. on a 6 more than one child is born (determined in further rolls how many exactly). on a 1 the child is a miscarriage (further rolls determine whether only the child dies or also the mother).
Adoption is also a possible way of getting children although a not so good one. While normal children will have certain chances to inherit traits from their parents, adopted kids are randomly determined and might be absolute monstrosities when grown up...and once you have them they are yours.
Sucession
again anything goes. However you have to settle down on one way of sucession and may not change that for two generations of rulers.
House Morgarten for example would be closest to Cognatic Primogeniture (male-female equality closest kin to ruler inherits)
Houses like de Treming and Hartwing new on the stage could start completely different ways of inheritance, like Elective or Tanistry.
some ways of inheritance in case you are not familiar with noble inheritance:
Primogeniture (Firstborn inherits, after that its the closes to kin, so secondborn son, thirdborn etc. until we have to go to brothers and uncles or sisters and aunts of the ruler)
Ultimogeniture (the opposite, the youngest son inherits, followed by the next oldest son, after that its the youngest brother, the youngest uncle etc.)
Seniority (the oldest member of the family inherits)
Tanistry (you vassals vote for a member of your family)
Elective (your vassals and your family members vote for anyone from the family or even somebody outside the family)
agnatic (Only males)
agnatic-cognatic (male-preference)
cognatic (equal opportunity)
uterine-cognatic (female preference)
uterine (only females)