It was just a phase.
A silly, momentary lack of sense, a non-permanent phase that will soon be over with because while he was a mudblood, he was also quit fit and Narcissa could see what Andromeda saw in Ted Tonks.
But it was just a phase.
A silly fling with a silly boy that would be over and done with and no one need know that anything had ever happened.
At least that's what Narcissa had been telling herself every time she found herself writing home or to Bellatrix. She found herself writing about the most asinine things, drawing out her letters to multiple sheets of parchment (that she doubted anyone read the whole way through anymore) trying to summon up the fortitude to mention Andromeda and her mudblood.
Three months of watching and biting her tongue and sitting on something that could only end in yelling and tears.
But, Narcissa reasoned, there was no need to bring wrath down upon Andromeda because soon her tawdry affair with the mudblood would end. Then there would be no reason to upset everyone. Not when the situation between her beloved sister and the mudblood was only temporary.
It was just a phase, after all.