The KML support [2.2] and Custom Map Service [1.4] probably do some of what you want … but they are virtually undocumented. So they need an explorer. One who will lead discussion of them here on the support forum, then expand the wiki with what they learn.
And the Time Lines Map addon Geography view has some custom markers… similar to what you request [1.2]. Exploration of its limits would be welcome.
The Sync Association addon tool [3.1] creates coresponding symetrical and asymetrical Associations. It could use some improvement for customizing and limiting the scope of its action. There is another thread suggesting an interface for simultaneous access to both sides of the association. Join the discussion and help resolve flaws in the proposed interface. (Flawed designs are less likely to capture the interest of volunteer developers.)
[7.1] keep an eye on the development of the experimental FamilyTreeView (FTV) for a diagram of direct relationships between people. It has been in alpha testing for the last few days.