Children with an older brother have poorer language skills than those with a big sister:

https://digest.bps.org.uk/2019/08/23/children-with-an-older-brother-have-poorer-language-skills-than-those-with-a-big-sister/

https://journals.sagepub.com/doi/abs/10.1177/0956797619861436?journalCode=pssa


https://srcd.onlinelibrary.wiley.com/doi/full/10.1111/cdev.13113

https://www.cambridge.org/core/journals/journal-of-child-language/article/minding-the-gaps-literacy-enhances-lexical-segmentation-in-children-learning-to-read/18BFE964409474318C6A44C464905D00


https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4064252/