Variation on a theme here:
Lets say Keitaro and Motoko were dating since early on in the series, and wanted to tie the knot, but were afraid their families wouldn't go for it. At first, they were quite despaired by all this, until Motoko got leaked info about her sisters visit, and Motoko had an idea.
They would just lie about their engagement. Tsuruko would naturally be distrusting on any sudden declaration, so to force a response, they would fake some obvious signs of not being a real couple, knowing that Tsuruko would force the issue to make Motoko an honest woman.
In other words, tricking her into organizing a shotgun wedding, with some tough alternative to getting out of it.
So it ends win win for them. Win and Tsuruko would basically acknowledge Keitaros worth and be okay with a relationship. Fail, and Tsuruko would force them to marry, and end result they are perfectly okay with.
I'm thinking Tsuruko actually falls for it, and throughout the fic, you think it's just a retelling of Burn up blade, with a few winks and nods that something is going on. Tsuruko only learns the truth when they leave a thank you gift behind in the end, which to her shock, is a photobook, filled with photos of Motoko and Keitaro's dating life, revealing that it was all a setup, and they were a legit couple.