Bro is my favourite preposition in Europic. All prepositions are categorized as a sub-type of adverb, and they (mostly) consist of a single CV or CCV syllable. So it really was a moral imperative to make bro a specialized preposition meaning ‘related to’ or ‘associated with’.
I already briefly mentioned the ‑la suffix a few days ago bro the word katri-la ‘strange’, but I should elaborate that ‑la is a sort of “specialty” affix, along with its nominal counterpart ‑lu. The affix ‑la converts a word into an adjective, while ‑lu creates a noun. Normally, this is done by either changing the final vowel to ‑a or ‑u (e.g. wecku ‘poison’ ↔ wecka ‘poisonous’) or adding an adjectival suffix like ‑alya ‘having a tendency towards’ or ‑osa ‘‑ful’ or ‑eca ‘-like’, or adding a nominalizing suffix like ‑itu ‘‑ity, ‑ness’, but these only apply to standard verbs, adjectives, and nouns: All other “oblique” parts of speech use ‑la and ‑lu for these purposes (e.g. bi ‘beside’ + ‑la → ‘adjacent’, bi + ‑lu → ‘side, flank’). (The original proposal for Europic even used ‑la to create possessive pronouns, like **mu-la ‘my’ instead of current ma, and ma-lu, while rarer, still means ‘mine’) Hence, bro ‘related to’ + ‑la → ‘related, associated’, while bro + ‑lu → ‘relationship, association’.
Another useful suffix often added to prepositions is ‑ke, which generally converts a preposition or other adverb to a conjunction, or converts a phrasal conjunction to a clausal one. (More about that another time, maybe.) It is also used to convert an interrogative correlative into a relative one, but that’s also for another time. In this case, it converts bro-la ‘related’ into a conjunction that basically means ‘relatedly’, but there isn’t a great direct English translation for all of its senses; for instance, another definition of bro-la is ‘respective’ or ‘in order’, hence bro-la-ke can also mean something like ‘respectively’.