I don't actually ship Susan and Bebe. Susan is just fine being married to Boyd, and Bebe Hart although she ages to YA quite soon, she can still have Ethan Bunch or Michael Bachelor if prom comes soon. If not, there are plenty of young adult sims like Christopher Steel or Stiles McGraw or Leighton Sekemoto or Connor Frio that could be perfect for her! And in the possibility that Bebe Hart might be lesbian, well there's Holly and River who are perfect for that. Well and Tori Kimura, Monika Morris, and Emma Hatch for examples that start up as young adults. And for Susan, as I said before she's perfectly fine being married to Boyd. When Boyd dies, Susan propably would'nt have long to stop mourning and find another person to date. (The premade sims tend to die of old age in my games - i often am too much of a cinnamon roll to kill them off) And if they divorce\separate\aren't in the same household for some reason, usually Boyd gets wishes to kiss and date other people, but Susan does not.
The Susbebe thing was mostly a whim I had sometime ago. Like a crackship. Susan most likely only likes Boyd Wainwright (and maybe Cycl0n3 Sw0rd and Liam O'Dourke) THAT way. Bebe is just another one of Blair's ex-classmates to Susan.
And to Bebe, Susan is just a random older woman that happens to be Blair's mom. Bebe would most likely prefer being Holly's stepmother via Nick than being Blair's stepmother via either of her parents.
I do plan to bred Susan with other sims that are not Boyd, but that's either in AUs or... because the Fovix-Nyaat made her do it.