3 and 8 are supposedly friends – that is what He Tu says.
If we view it from Trigrams and 5-element perspective, 3 is Zhen and 8 is Gen trigram.
3 is Wood and 8 is Earth.
Wood counters Earth. Hence, Zhen counters Gen.
So, Eldest son counters Youngest son. The logic is purely based on trigrams interpretation.
Zhen and Gen can also be seen as Zhen over Gen which gives us the Hexagram image of Thunder over Mountain. The gua name is Lei Shan Xiao Guo (雷山小过).
Mountain is big but thunder is small. In Yi Jing, this implies there is a little excess, a little surpass of what it is supposed to be.
The da gua image of this is Kan. Kan is water and it usually denotes unforeseeable danger and some mistakes. Kan also depicts Yang line is trapped in the yin. The man is sandwiched between the women.
3 and 8 can be 3 + 8 and therefore gives us 11. 11 is a master number in the study of numerology which sums up to 2.
2 is about loving, harmony and togetherness. But 2 is also about being over-sensitive since 2 is associated with moon which waxes and wanes. The mood goes up and down. This can easily lead to disputes and conflicts. In Vedic Astrology, the moon controls our thinking and emotions. It is the fastest shifting planet.
This 2 is however derived from 11. 11 can bring emotional baggage since this is a master number.
If there is an issue here, the best way to tackle and dissolve it is not to play it up.
Yi Jing would suggest a conservative approach to be taken. Inaction is better than action. To retreat is better than to advance.
Down play the issue is deemed to be auspicious. Upsize it and one is in danger.