This was actually just discussed earlier today. It would appear that other players have already interpreted that rule to mean immortal blood line only. So, adoption is fine for other non immortal Sims.
What Ratchie is referring to was said on
Page 233.
I don't mean to contradict your rules Metro; this is your challenge, so your rulebook, with a very big period at the end of that. However, following the quoted statement, as well as your reply to my PM OK'ing sims not related to the Immortal bloodline to adopt or conceive without both sims living in the house (which I can quote if you want, just let me know), I went ahead and planned the first portion of my Dynasty. To see that all go up in smoke if I choose to go for HoF (i.e. following every rule to the letter), is more than a little disappointing. Which I don't blame on you at all, it's just a sticky situation where some choices will have to be made.
Different applications of this rule to Immortals/non-Immortals have occured in many Dynasties for a while now. Especially considering that how you are currently explaining the rule, non-Immortal male sims can't have children with female sims not living in the house either.