Highguard here! The way it’s discussed in play it feels about 50/50 that you’d be in a chapter with blood relatives.
Some small settlements might only have one chapter nearby, so people wanting to join one would all join the same one. In large settlements, there are multiple chapters, so you then choose based on things such as chapter ideology or intent.
Chapters have a very family feel to them, so if you are in a chapter and have children, it’s likely those children would be raised in the chapter. Once of age those children could take the chapter oath, or find a chapter that suits their interests.
In play its evolved a little that casual relationships between two people of different chapters are no big deal. However, serious relationships usually involve meetings between the respective exarchs of the two chapters, and Maya results in one of the couple joining the other’s chapter.