From a book review in Amazon:
英語を教えている関係で、人はどうやって話せるようになるのか興味があって、手にとった一冊。
A book I picked, because I am concerned with teaching English and interested in how people learn / become able to speak.
Although DIJG defines N+の関係で as "because of", my gut tells me that 関係 means connection/concerning and で adds the "as/because/since".
英語を教えている関係で、人はどうやって話せるようになるのか興味があって、手にとった一冊。
A book I picked, because I am concerned with teaching English and interested in how people learn / become able to speak.
Although DIJG defines N+の関係で as "because of", my gut tells me that 関係 means connection/concerning and で adds the "as/because/since".

As you already noticed, 信頼 has shades of meaning, so it (edit: 信頼しあってる) might be 'mutual trust/dependence', 'confidence/faith in each other', etc. depending on the context.