I still have a Joomla 2.5 site running because it uses a component called Joaktree which handles Genealogy data really well. In fact I haven't found anything better.
There is a beta version on Github at https://github.com/joaktree/J3.x. but the author hasn't supported it for some time so I guess he is ill.
I have two questions: 1. I don't know how to install the software from that location as installation files for Joomla are normally zipped in some format. Can someone advise? 2. I guess it is feasible for a non expert coder to slowly work through the errors and fix but does it make sense to do so?
you may take content of https://github.com/joaktree/J3.x/tree/master/com_joaktree folder in repository and copy it over Joomla root. Then go to Joomla menu -> Extensions -> Manage -> Discover. Click Discover button in toolbar. It should find your extension. Select it and Click install and it will register it.
Same with modules and plugins.