On a side-note; I am warming up to the idea you had that Shannon and Jess are somehow related. A lot of things would make a bit more sense if that ends up being the case.
What theory exactly? That one of Jess' parents had something with one of Shannon's parents? They wouldn't even have to be half-siblings or anything. I'm imagining a scenario like that: A parent of Jess (henceforth eJ) and a parent of Shannon (henceforth eS) were at a school event. The other parent didn't show up, for whatever reason, and eJ and eS get to talking at some point. The chemistry is good so far, the event ends and nothing happens. The two meet, randomly, but every now and then and talk and they decide to go out for coffee together. At this meeting, there may be a major downpour and the two may share an umbrella or one may drive the other home because one of them had come on foot or by bus. And at some point it comes so far that the spark jumps over and romantic feelings arise.
However, whatever the reason may have been that they both couldn't stand each other, we agree that it must be something that either embarrassed/uncomfortable them both or that could make Frank possibly think worse of them.
If there really was romantic interest on Shannon's part for Jess, I think it might have played out similar to the scene where Lilli gave Frank a nice black eye:
Shannon confronts Jess, Jess gives her an ultimatum. Unlike Lilli, however, Shannon says no deal and the two have been at each other's throats ever since.
We don't know what year Single Again is set in, but I would assume around 2019. That would mean:
Shannon, Frank and Jess were born sometime around 1980.
Abby and Lilli around 2000.
Becca around 1996 and Meiko around 1995.
The year Meiko was born must also be around the year Frank moved and met Jess and Shannon.
I did some research and apparently Canada had a fairly early advancement of LGBT rights, so my thesis is not necessarily correct. At the same time, we know very little about Jess and Shannon's parents, so it may well be that they were religious nuts, or outwardly liberal and open, but if suspicion had fallen on Shannon or Jess, that would have changed quickly.
Who has time and leisure can look if somewhere is mentioned for how many years Vanessa and Beth are married, since when same-sex couples are allowed to marry in Canada and what that says for our given timeline. Maybe we will find a new clue or new theories what the problem of the two was with each other.
And I think there is hardly a picture of Becca that shows her better in her role as an unpredictable wild card. Also, I think I need to make a collection of my favorite scenes with Lilli. There are some heartwarming and emotional scenes with her.