This study investigated the various linguistic, psychological, and sociocultural variables that explain the sociolinguistic variation in English as a Foreign Language (EFL) among Arab siblings. The variables are not predetermined but extracted from the participants' contributions and data in a grounded theory research. The siblings participated in this study (N=15) are university undergraduates, who are educated in the UAE schools. While all human beings are empowered with the Language Acquisition Device (LAD) proposed by Chomsky, this does not explain the sociolinguistic variation among people and siblings in particular in using the language. The innate ability is not also evidence that siblings will have similar proficiency levels. The discourse analysis showed noticeable sociolinguistic variation among siblings in terms of language accuracy & fluency, L1 interference, spontaneity, and automaticity of speech. Some of these siblings have equal opportunities for improving their English; however, the analysis showed that some affective variables influence the development of their proficiency levels. The thematic analysis signaled three variables namely exposure to quality input and interaction opportunities, provision of quality education, and autonomy and independent activities. Eight of the fifteen participants attributed their deficiency in English to the inefficiency of the education system that did not facilitate sufficient exposure to linguistic input. Autonomy and independence proved to be effectual in creating the variation among siblings as well. This study sheds light on the significance of contextual factors in influencing proficiency levels among EFL learners in the UAE. The implications and recommendations of research findings are discussed.