The location of Domus Lia is more than great,Piazza San Pietro is like 5 minutes away:) (Piazza del Popolo is also not so far away, by foot).
The apartment is great, I will definitely recommend it (for a family stay or otherwise). Stefania is a great host, definitely helped with her advices, and did us a huge favour with keeping our luggage in her apartment for a few hours (thanks Stefania:) )