Yep, Ralph seems more than happy for the MC to bang his mom… Will that be the case if you actually succeed?
In the current version, depending on dialogue choices. Ralph will flat out tell you to stay away from his sister. Kim also suggests Ralph wouldn’t be happy with you two hooking up. Although you can’t get both those scenes in one play through as they’re on opposite paths.
Ralph is an important character in so much as he can cover for you or do other things you might need. If the relationship level drops then he’ll be less likely to do stuff for you.
You’ll be able to hide any potential relationship with Kim from Ralph, but that may require sacrifices. Things such as public displays of affection etc…
There will also be the option to not care what Ralph thinks. You may then need to make friends with other male characters or utilize female characters.
Or you might be able to get Ralph on side and him be ok with you banging his sister. What might Ralph want that you can arrange….
Pregnancy would be further down the road, if at all. It doesn’t fit in the first part of the story, and you have to get the MC laid first!!