Your crime archives link is Ted "never met a conspiracy I couldn't embrace" Gunderson and the fella (Mr. Semmes) mentioned is Ted's friend.
According to my brother (who is also a lab tech and supervisor), you can analyze urine - even back then - for antigens and antibodies. In the 1970s, he tells me, DNA was not available. But for antigens and/or antibodies - yeah, you could do it.
So, what does that mean? It means that if I were a lab technician back then (and not a pre-teen), I could eliminate certain people as contributors of the urine. The father and the youngest daughter lack the antigens/antibodies and can therefore be eliminated. (That means they could not have done it.) The other two people known & proven to be in the house (their bodies were found in it) cannot be eliminated. I may not be able to say definitively that Kim was the source of the urine, but I can say it's possible she did it. If no urine stains are found on Colette's pajama bottoms, but there is a urine stain on Kim's nightie, a reasonable person would conclude Kim did it.