Hi, when you interact (E button) with the mom you should get a UI showing 2 options: Fuck or Use Object.The fuck option is locked on the beginning; you have to unlock it stretching mommy's holes to the max level.
I kept playing and figured it out naturally...
Only one issue I came across was pulling out during vag sex caused a total freeze.
Waiting for updates... I like where you're at... Maybe hide a strap-on in the dad's stuff for possible DP...