In the half a millennium since Shakespeare was writing, English has evolved new and different rules of grammar. In early modern English, relative pronouns were omissible when they represented the subjects of relative clauses. This is not possible in modern standard English.
This has nothing particularly to do with the fact that Shakespeare was writing in verse. It was just the standard English of his day.
In modern standard English, when a relative clause contains an expressed subject, we can omit the relative pronoun (except in the case of whose). In such cases, the relative pronoun most often represents the object of the relative clause. However, when the relative pronoun is co-indexed with the subject of the clause (or actually is the subject, depending on how you interpret the grammar) it is not possible to omit it.
- That is the woman (
who) he loves.
- *That is the woman (
who) loves him. (ungrammatical).
In the example above above we see that in (1), where who is interpreted as the object of the relative clause, we can omit it. However in (2), where who would represent the subject of loves, it cannot be omitted. [Note however, In some non-standard dialects of English (2) would be perfectly grammatical].
However, this is only true about present day standard English. In early modern English it was very common to omit the pronoun is sentences such as (2), probably more common than to omit them in sentences such as (1). In Shakespeare's day the omission of relative pronouns was becoming rarer, but still occurred. He himself used bare relative clauses without pronouns reasonably frequently.
Here is the pertinent section on early modern English from The Oxford English Dictionary in their entry Grammar in early modern English:
Relative pronouns. The relative pronoun that remained common (as it still is), but a number of alternatives existed during the period. the which was inherited from Middle English but became rare by the mid-seventeenth century. which could be used for both persons and things but became rare for persons after 1611. who as a relative pronoun was rare in the fifteenth century and gradually became commoner in the period. The use of the so-called ‘zero relative’ (i.e. no pronoun at all) arose in Middle English but was rare in the sixteenth century. In the early modern period it could be used where the relative was the subject of its clause as well as object (now largely non-standard or poetic), e.g. ‘Life it self..is a burden [zero relative] cannot be born under the lasting..pressure of such an uneasiness’ (John Locke, 1694).
In his book From Dialect to Standard: English in England 1154 - 1776 (2005), Hans Nielsen writes:
Other relative pronouns in early Modern English were as and the zero relative, cf. the following examples taken from Barber (1976: 220): those _as sleepe, and thinke not on their sins and My father had a daughter _ lou 'd a man. In the latter example the zero relative is in subject position, which is no longer possible in written Standard English. However, the zero relative could also occur in object position as today, cf. ... the Vow ___ I made to her in Marriage.
As is intimated but not specified in the extract above, it seems to have been more common for relative pronouns to be omitted when representing subjects than objects at this time.
In his book A Shakespearean Grammar (1869), accessible here, A. E. Abbott notes of his examples of zero relative pronouns:
Most of these examples (except those in which when and why are omitted) omit the nominative. Modern usage confines the omission mostly to the objective.
Noting that in 'many cases the antecedent immediately precedes the verb to which the relative would be the subject', he gives the following examples:
- I have a brother is condemned to die. (Measure for Measure)
- I have a mind presages. (Merchant of Venice)
- The hate of those love not the king. (Richard II)
- In war was never lion raged more fierce.(Richard II)
- And sue a friend 'came debtor for my sake. (Sonnet 139)
- What wreck discern you in me deserves your pity? (Cymbeline)
- You are one of those would have him wed again. (Winter's Tale)
The Original Poster's question
The Original Poster asks whether we could say:
- I have a brother condemned to die.
The answer is 'yes'. We can use participle clauses like this to post-modify nouns in English. Some writers argue that these are a form of reduced relative clause formed by deletion of the both subject and the verb BE.
In any case, the first example:
- I have a brother is condemned to die.
... as suspected by the Original Poster, would not be considered grammatical in standard modern English.