Is using the relatives’ friend’s recommendation inappropriate?
As a rule, where a significant personal connection exists between a referee and the subject, there is a potential conflict of interest. Definitions of "significant personal connection" are not universal. Policies on the acceptability and handling of potential conflicts of interest are also not universal. Where a potential or actual conflict of interest exists, it is imperative that it be declared.

A referee who has a significant personal connection to the subject may still be acceptable, if he/she has experience of working with the subject in a professional capacity, and the personal connection is declared. However, it should be understood that such a referee's comments would carry less weight than an equivalent referee (in terms of stature and experience of working with the subject in a professional capacity) without a significant personal connection. In some cases, it may be that such an "equivalent referee" may not exist, and so the referee with a significant personal connection is the only feasible choice (although, as I said in my answer to the related question, if more than one of the referees had a significant personal connection, that would be a red flag, since it would imply that the subject relied on friends to compensate for lack of merit or initiative).