I acknowledge that my idea doesn't have any real support. It's more that it's hard to prove wrong and it would be awesome if it turned out to be true. But true or not I'm already working on fanfiction ideas.
But there actually is some proof of Lynette NOT being connected to the Royces. The conversation with Jill's parents on her path in episode 10 reveals that the Royces bought the house about 19 years ago, that the "previous owners" sold it cheaply because of financial issues and they got the tip from Bianca's father Maynard, who while a friend now was merely a business associate at the time.
If the Royces knew Lynette's parents why would getting a tip from Maynard be so significant and why refer to their friends as "previous owners"? Why not mention having visited the house for years before buying it as they surely must have done if there was any connection beyond the house itself?
Also, while Miranda and Viv look a bit too much like Bianca and Clara (or is it Clair? I can never remember which is which) to be a coincidence, they should already have been born at the time of the diary events, so I'm more inclined to believe they are aunts rather than mothers. But whether Miranda is Maynard's wife, sister or sister-in-law, he is in an excellent position to give the Royces a tip on the house Lynn's parents are selling and thus Bianca is the character connected to Lynette rather than Jill.