The confusing-ness comes from a few factors: (1) the fact that commas are very commonly used to mark an appositive phrase, so the second part initially looks like it's an appositive (meaning that it elaborates on the first part); (2) ellipsis of this extent is not so common in modern writing, so less experienced readers will get confused; and (3) it isn't so common to put more than two full clauses in a comma/and list.
If I were trying to reduce confusing-ness, I would reduce the extent of ellipsis, break up the parts into two sentences, use semicolons, switch to plural 'relationships', or directly number the parts:
* There is typically an avoidance relationship between a man and his mother-in-law, and usually one between a woman and her father-in-law. Sometimes, there is one between any person and their same-sex parent-in-law.
* Avoidance relationships exist (1) typically between a man and his mother-in-law; (2) usually between a man and his mother-in-law; and (3) sometimes between a woman and her father-in-law.
* Avoidance relationships (1) typically exist between a man and his mother-in-law; (2) usually exist between a woman and her father-in-law; and (3) sometimes exist between any person and their same-sex parent-in-law.
Let's try it with a zip() function on two lists! This is better, right?
"There is an avoidance relationship between a man and his mother-in-law, a woman and her father-in-law, and any person and their same-sex parent, typically, usually, and sometimes, respectively."
If I were trying to reduce confusing-ness, I would reduce the extent of ellipsis, break up the parts into two sentences, use semicolons, switch to plural 'relationships', or directly number the parts:
* There is typically an avoidance relationship between a man and his mother-in-law, and usually one between a woman and her father-in-law. Sometimes, there is one between any person and their same-sex parent-in-law.
* Avoidance relationships exist (1) typically between a man and his mother-in-law; (2) usually between a man and his mother-in-law; and (3) sometimes between a woman and her father-in-law.
It feels like refactoring in programming!