These tips are very basic but I remember my first playthrough and got the Farmer ending one time and I knocked up my Sister so yeah...
Yeah, I got the farmer ending first time around as well (I think I tried too late to get my sister's help with the guild, so the event wouldn't trigger). It's going better on this playthrough, but I did notice last time that I tried doing some lewd stuff with her at night and she'd invariably wake up or I'd run out of time before I could get anywhere, even though her lust and interest were both decently high (70+ for each, IIRC) and I had the perks to reduce the chances of her waking up and increase the positive effects of touching. Also, the text seemed to indicate that she was in to things (something like "Ah, brother, just be gentle!" when she woke up), but in practice she still didn't seem to like it (everything increasing the Bad meter once she was awake), which threw me for a loop on what I should be doing.
Preggo 'enjoyers' are straight crazy.
I think it's like most kinks/fetishes: most people who enjoy it are pretty chill, but you'll always have a vocal minority who can be loud and shitty about not getting what they want. Personally, I more often see people getting really upset about the presence of kinks they don't like than the other way around.