The ability to communicate with others is integral to success throughout life. Thus, it is imperative that young children acquire language holistically and effectively. The presence of older siblings plays an important role in a child's language acquisition and cognitive development. Research has shown that the presence of older siblings has a negative effect on infant language development, likely because the language input from older siblings is of lower quality than input from adults (Havron et al. 2019). However, this effect is subject to variations as a result of the age and gender of the older siblings (ibid.). In my project, I will investigate the “sibling effect” on infant language development in the first two years of life, using a corpus of data recorded by Language ENvironment Analysis (LENA) software. The data consists of audio recordings of 24 English-speaking infants' weekends, collected at the ages of 6, 10, 14, 18, and 24 months. LENA automatically identifies language input variables such as the number of adult words that the infant hears (Adult Word Count, AWC), and the number of linguistic vocalizations that the child produces (Child Vocalization Count, CVC). In this project, I will investigate the effects of CXN (other child speech), FAN (female adult speech), and MAN (male adult speech) on the CHN (key child speech). I will also track correlations between this and the number of words each infant is able to produce at each age according to parental reports (The MacArthur-Bates Communicative Developmental Inventory; Bates, Dale, and Thal, 1995). Preliminary findings suggest a weak negative correlation between CXN and CHN as well as the number of words each infant is able to produce. Extant literature has demonstrated a negative effect that is sensitive to a variety of other independent variables such as gender. My research will inform parents and early childhood educators as they oversee and optimize the language acquisition process of children in families with older siblings.