I am living with my brother, his wife, and their one year old daughter (my niece). They are attempting to raise their daughter bilingual in two languages, Language A and Language B. Language A is the local language, and all of us adults in the home are native speakers of that language. My brother and his wife are advanced near-fluent speakers of Language B, and I am a beginner in Language B. I am an advanced speaker of Language C, which neither of them speak. The languages all belong to different language groups and are not mutually intelligible.
Can I learn Language B alongside my niece? Have there ever been any studies on children and adults learning a language together? Are there any best practices?
I am currently learning using books and videos and making attempts to speak to my niece using both languages (saying something in Language A, then immediately translating it into Language B for her to the best that I am able). She is still in the single word language stage, but is picking up words from both languages (and preferring one or the other for different ideas).
Part of me thinks that I should just learn the language like any other adult, and things will develop naturally, while another part of me says that I should concentrate on learning vocabulary relevant to her and with her, possibly becoming a high beginner or mid-intermediate speaker who is more comfortable talking about sparkle pastel glitter ponies and princess dress-up than politics.