Kabbalah zu Wajikra 18:6

It also says “each man should not come near his relative to reveal their nakedness” (Leviticus 18:6), and from here they say “one should not gather with only women, and outside his city even with his sister, daughter or mother-in-law, not to give rise to people’s suspicion”. Regarding relatives it says “do not come near” (plural) and regarding a man with his Niddah wife it says “do not come near” (singular), since it is coming to include anything that might lead to a sin, since one should distance himself from anything repulsive and anything similar, as the sages say “distance yourself from a minor sin since it may lead you to a major one, and run towards a minor Mitzva since it will lead you to a Major one”.