The affix cha (īya) is added without a change of sense to a stem ending in the word jāti when it means appropriateness.,
The word बन्धु means the substance or substratum in which jati or generic quality adheres or any thing by which a thing is regulated and individualised (बध्यते नियम्यते स्वतन्त्री क्रियते). Thus बन्धु means that substance by which the genus Brahmana, Kshatriya &c is distinguished. Thus ब्राह्मणजातीयः \appropriate to a Brahmana\. क्षत्रियजातीयः, वैश्यजातीयः &c.
Why do we say \when meaning appropriateness\. Observe ब्राह्मणजातिः शोभना \the Brahmana caste is beautiful\.,