Alternatively, she just likes to draw very horny lesbians.
This is the answer. The pin ups are just that, pin ups, no real connection to the doujins. Yohane's pics are for the most part based on questions asked to them on their website, like "I want a book where Sanae enjoys an oyakodon of Previous Generation Miko-sama (37 y.o.) and Reimu-san!", and thus draws Sanae with a pregnant Miko and pregnant Reimu.
There's no "all these relationships will die eventually in favour of sex" headcanon, they just like to draw porn and ships. A lot.