I will try to answer why Spike is regarded as the elder brother of Snoopy, while Marbles / Olaf / Andy / Belle are regarded as the younger brothers and younger sister of Snoopy.

Actually, Mr. Charles M. Schulz did not say who is elder and who is younger.

However, some people believe that Spike is older because of the following reasons.

(1) First appears in the comics strips:

Spike 1975 Belle 1976 Marbles 1982 Olaf 1989 Andy 1994

(Molly and Rover never appear in the comics strip.)

(2) When Snoopy is getting married in 1985, he only invited Spike.

We always think that one must invite his seniors to his wedding.
(3) By counting the number of strips, Spike appears the most frequently in the comics strips among the Daisy Hill Puppies (of course besides Snoopy!) He appears on 481 days.

我嘗試解答一下為什麼Spike 是大哥。

其實舒爾茲老師一直沒有表示誰是哥哥、姊姊、弟弟、妹妹。

Spike被認為是大哥主要原因：

J他最早出現在漫畫

J1985年Snoopy打算結婚，只通知Spike來做証婚人，咁一般認為，做証婚人一定是長輩。

所以，Marbles Olaf Andy Belle被認為只是弟妹。

J另外以數量計，Spike是除了Snoopy以外，最多出現在daily Comics strips, 合共出現了481日。