From observation, what I have seen is.

Please use a Vedic/ Sidereal chart to confirm!

There’s no younger sibling if the Sun is in the 3rd house or Venus is in the 2nd house. If both are seen together in a chart, there’s no younger sibling.

• The Sun burns the LIVING signification of that house, which means there’s no younger sibling, but the person will enjoy travel.
• In indian mythology, the right eye of Venus is blind. Therefore, we will not see the living signification of the second house from the house where Venus is placed in a chart. It can indicate physical distance. For example, if Venus is in the 8th house, the person lives far away from her father (does not see her father often). The father is there, of course, because one needs a father to be born. Venus in the 10th house, there’s no elder sibling. Venus in the 2nd house, I haven’t seen the younger sibling yet.
• And Jupiter's aspect on the 3rd house can change the game. Always check where Jupiter is before predicting!

Other malefic planets such as Mars, Saturn, Rahu, or Ketu in the 3rd house DO indicate the presence of younger siblings.