Siddham is one branch of a very large family. The Brahmi of the Gupta period gave rise to a set of related hands across northern India, and from those descend Devanagari, the Nagari and Ranjana scripts of the Himalayan region, the Newa script of the Kathmandu valley, the Bengali and Oriya scripts, the Tibetan script and, further off, the scripts of Southeast Asia. Siddham is a sibling of these rather than an ancestor.
The family resemblance is easiest to see in structure rather than in shape. All of them are abugidas, all inherit the same order of letters, all write vowels as dependent marks, all stack their consonant clusters, and all treat the syllable as the written unit. A reader of one can usually work out how another is put together long before being able to read it.
The differences are largely a matter of when each branch left the common stock and what happened to it afterwards. Tibetan carries adaptations for a language with sounds Sanskrit does not have. Devanagari carries centuries of subsequent adjustment for the languages of northern India. Siddham carries almost nothing after its departure, because it left early and was preserved rather than used.
Reading them side by side is how the shared order becomes visible as an inheritance rather than a coincidence.