Welp the friends discovering the whole thing was just a warm up it seems
To defuse the situation :
A/ This is a misunderstanding, we were only pretending =)
B/ This is a misunderstanding, there is nothing between us, i was merely forcing myself on imouto-chan, no worries =))
C/ There is no misunderstandings, we are in a relationship tehee
Might be an instance where the father gunning the whole family before shooting himself could be a viable option.