I wouldn't worry about it. The wife replies that Mu-kun is the first absentmindedly; then, the aunt brings attention to that, by asking (confirming) if he's the only one. Wife realizes what she's said and gets embarassed. Dunno why, maybe the other readers were right and it's a culture thing. Then she says to keep it a secret (that Mu-kun was her first and only) and Aunt readily agrees. If it was a case of cheating, and therefore an estabilished secret between the two of them, then she wouldn't have blushed like that or stressed in that way that it was a secret.
Oooooor I could be wrong, and she cheated. That's very unlikely, tho. Again, I wouldn't worry about it. Series is completed, in any case, so perhaps someone already knows.