More so out of convenience if they all agree to try a throuple closed poly relationship, while still not necessary, the fact is Des is currently living in the dorms, sharing a room with a roommate, so there would be the added advantage of discreet, taboo sex, non-stop whenever the mood strikes, which if Mom is around, that will be often the case.
Whether Mom is the biological or just step mother, the fact that she raised Viv would probably not change whether she was attracted to her or not. Remember she's already had experience with taboo/incestual relationships with her in-laws' family. That said, I'm sure she'd wait as long as possible (in this case, Viv is close to college graduation) without her departed husband around to spur her on to make a pass at her. The fear of course is how Viv would take or react to her 'mother' perving out on her and attempting to seduce her. Which is why Becca notwithstanding, Mom has more than enough lovers to relieve that sexual tension, despite the heart, wanting what the heart wants... err, rather loins.
The only way I can see if Mom were to consider making a pass at Viv earlier would have been if for lack of a better example, Viv were more similar to Eva's personality with the obvious promiscuity, especially with women. While Mom admitted she's always been attracted to girls and even accepted she was probably gay, until she met her husband, in contrast, Viv at the start of the story, is a bookish, nerdy, heterosexual girl (without Tara's intervention) with a modest to almost non-existent sexual appetite. It would scare off Mom enough to not even want to take that chance and lose her daughter forever, just to pursue a moment of lust with clearly no romantic long-term intentions.