What in a chart might indicate an overshadowing sibling?

Third house rules siblings and is associated with Gemini. Pluto is intense, fated relationships and powerful responses. Alternatively Neptune undermines in a more subtle way. Look for squares and oppositions to personal planets either from or into Gemini or the third house. There would normally be more than one indicator in a chart if this was a significant situation in the lives of the siblings. You could also do a Davison Relationship Chart or a synastry chart, but NEVER a composite - at astro.com