Translation:Aunty Lani is the birth mother of my cousin.
I don’t think it’s an exception, rather I think it’s a matter of understanding the Hawaiian mindset about family. According to Pukui and Elbert, it’s actually the other way around: lūauʻi is the noun. So makuahine is actually the modifier, I.e. instead of “natural mother” it’s actually something like “mother-role natural-parent.”