Disclosure: I work for GM. Anything here is solely my own opinion or experience.
Fun fact: Chevrolet kind of bought GM. https://www.history.com/this-day-in-history/gm-buys-chevrole...
> "Still the owner of a considerable portion of GM stock, Durant began to purchase more shares in the company as his profits from Chevrolet allowed. In a final move to regain control, Durant offered GM stockholders five shares of Chevrolet stock for every one share of GM stock. Though GM stock prices were exorbitantly high, the market interest in Chevrolet made the five-for-one trade irresistible to GM shareholders. With the sale, concluded on May 2, 1918, Durant regained control of GM"
And then after this the DuPont Family effectively controlled GM for quite a while. They previously had a relationship because GM used DuPont paints.
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As to the manufacturer family tree: things get significantly more complicated when you add joint ventures. Ford famously partnered with Mazda, Chrysler partnered with Mitsubishi, and GM partnered with Isuzu and Suzuki and even Toyota for a while (NUMMI). Nowadays GM and Honda have a few codevelopment projects; even the concept of what a joint venture means has significantly changed. My list is this comment is far from authoritative or exhaustive.