English Language and Islamic religious law are not really parallel bases. English has historically been a language of government and heavily influenced by Christian religions (the modern language got a jump start from early translations of the Bible). So most of the common words that relate to marriage reflect Christian or secular constructs.
Virtually all laws governing marriage, both secular and the major religions, have exclusions based primarily on kinship, but often flavoring that with additions and exceptions based on social mores or political influence. Under the umbrella of the English language, there are countless different religions and governments, each with different standards for what is prohibited.
So there are likely other words directly equivalent to mahram and Namahram used within their respective communities, but the English words tend to be more generic, reflecting the concept of prohibition based on kinship rather than "the" specific, actual prohibition.
"Consanguinity" is the generic English term for kinship or blood relationship. The degree of relative consanguinity is a measure of how closely two people are related. This can be laid out in table form and the steps of separation (degree of consanguinity) can be counted. The prohibitions of the different religious and secular laws governing marriage are roughly based on different acceptable degrees of consanguinity (See Wikipedia discussion here).
"Consanguineous marriage" is essentially a technical term for marriage of two people of the same blood or origin, i.e., descended from the same ancestor (see M-W). Like the term "consanguinity", it has degrees. However, people sometimes use "consanguineous marriage" as a shorthand way to refer to marriage prohibited due to laws based primarily on degree of consanguinity.
If I understand correctly, mahram and Namahram are terms for the people who would be marital partners. I'm not aware of individual, common English words meaning the people one is or isn't permitted to marry based on laws of consanguinity. That's commonly a religious perspective but doesn't correlate well with secular law. Conceptually, secular law looks at the marriage and whether the requirements are met rather than the perspective of each person.
English discussions of prohibited vs. non-prohibited marriage typically just use qualifier words like "prohibited" and "non-prohibited". I think the closest you will come to the English equivalent of mahram and Namahram is consanguineous and non-consanguineous, which at least get you to the general concept but would require different sentence construction. The alternative is words that are more generic or multi-word phrases as Harsh Sharma suggests.