Hi, what an interesting situation.
We've recently published a position paper on the role of L1 in EMI, which builds on research evidence and emphasises that students who are allowed to maintain mother tongue demonstrate better sense of identity and wellbeing, but – crucially – also perform better academically!
The paper can be downloaded here: https://elt.oup.com/feature/global/expert/?cc=gb&selLanguage=en
It's the one in the middle.
Good luck building your argument.
Have you thought of composing a petition letter for everybody to sign---to change your brother's school English only language policy? You could attach that letter here if you want. Another option is writing directly to the school's dean or headmasters explaining your reason (s) as to why the school must change the school-wide status quo language policy. Re: "Please send me any helpful studies." Response: I do not know what help you are looking for here, Collen. It may end up a research issue in which the pros and cons must be closely examined and proven. In your particular case, a negative diagnosis on the why an English spoken language only school-is harmful in the life of the school and its students. On vice versa, the positive argument of the study for supporting evidence.
Ofelia García is a renowned scholar of translanguaging. At the top of page 18 here, she and her co-author highlight a project in NY schools: "To highlight CUNY-NYSIEB's school-based work, we now turn to three examples of teacher development and student learning. In particular, we focus on how three different teachers adapted their pedagogy so that their Latinx students could build on their Spanish resources to access content and texts in English, develop their metalinguistic awareness, and cultivate a sense of pride in their bilingual, bicultural Latinx identities." https://ofeliagarciadotorg.files.wordpress.com/2019/03/observatorio-report-sustaining-latinx-bilingualism-ny.pdf
On García's website (scroll to below the books), you can download her articles and chapters. Note that there are also videos shared on her site: https://ofeliagarcia.org/publications/ A likely useful article there: https://ofeliagarciadotorg.files.wordpress.com/2018/01/vogelgarciatrlng.pdf
From page 9 of that: "Much of the translanguaging that occurs in classrooms is pupil-directed (Lewis, Jones, & Baker, 2012B). For example, in the U.S. bilingual kindergarten class described in García's (2011) study, students used translanguaging to mediate understanding, construct meaning within themselves, include and exclude others, and demonstrate knowledge, among other meta-functions. The edited volume by Gort (IN PRESS) also gives evidence of the presence of translanguaging in bilingual classrooms in the United States, whereas the contributions in the book edited by Paulsrud, Rosén, Straszer, and Wedin (2017) include examples of translanguaging in other contexts, mostly Nordic countries. Translanguaging as a practice in schools is being recognized by scholars around the world, whether in South African classrooms (Krause & Prinsloo, 2016) or in classrooms of Quechua in Peru (Zavala, 2015). Among the edited volumes and journal articles discussing translanguaging practices in educational programs around the world are Blackledge and Creese (2014), Cenoz and Gorter (2015), and Cenoz and García (2017)." This is the 2011 study mentioned there, I believe: https://ofeliagarciadotorg.files.wordpress.com/2011/02/biltranslanguagingkind.pdf
Pages 111-113 (Translanguaging in multilingual classrooms) highlight ways in which Garcia has found translanguaging by multilingual students to be valuable: https://repository.usfca.edu/cgi/viewcontent.cgi?article=1410&context=diss "In the book, The Translanguaging Classroom, Garcia et al. (2017) discuss various purposes for translanguaging that help to create culturally-sustaining environments. Garcia et al. (2017) state that translanguaging helps to facilitate stronger student engagement with complex content and texts. Their research shows that translanguaging gives students an opportunity to "draw on all of the resources for learning in their linguistic repertoires' (Garcia et al., 2017, p. 8). Students can collaborate with their classmates and teachers at varying degrees of proficiencies in multiple languages, gain new vocabulary, and a deeper understanding of the content through extended discussion and analysis." (p. 111)
All the best.