Eurail passes have changed since I last looked at them. They offer a second class family pass which is good value against the PTP 90 day in advance tickets I am considering. The price is a bit dearer but less mucking around booking tickets on net. How do the sellers of Eurail passes (and others who offer a family discount)make a judgement on whether you are a family or not? Eg the friend I could be travelling with could be a same sex partner; the 18 year old nephew could be an adopted family member. What proof do companies require?
If you look at the booking page for family pass, family defined as 2-5 people (adults and kids combined). Adults are charged full rate, kids 4-11 half rate, and below that free. So they don't care how your "family" is put together, you just have to provide the details of the people travelling together.