Hi. I don't get to this end of the boards often, but this conversation is such a great read.
Diyabolo, I am sort of in an opposite role from you; I am the English-speaking primary caregiver. I have two children and my dh is a native speaker of Berber and Arabic (and also speaks his very colonial French). Because of the stresses of circumstances and I suppose a lack of mindfulness, we now have two older children who are struggling to catch something in these languages. I have a little of each too, but a semester's worth of French and my very elementary Arabic are little to no help most days. We lived until recently in a suburban-to-rural area in US where English is all there is.
Fast-forward a couple years and we are now living in UAE, where Arabic is a school requirement. Regardless, quality education in Arabic is nearly nonexistent and where found, outrageously expensive. We've found an excellent tutor and pulled our kids from school there to work at home, so they will have more time and focus on their language learning. Two years in, my children are beginning to understand the gist of some overheard conversations, maybe some news reports. And this is all with the extra effort and time we put in. UAE is expressing a crisis these days, in that Arabic is being lost (to English) there. English is the primary language of instruction (although there are French and I think also Chinese schools), and the transient nature of most of the expat community means a lot of turnover both in teachers and students. This means that even in middle to upper grades, even with differentiation between native and non-native families, there are so many low-level language learners coming into class that nearly every year is a repeat of greetings, colors, shopping words, etc.
The struggle for us is that we know the cultural exposure is really good for our kids, and we want them to get the language, too. Arabic is a fascinating language to me, too. I study with the same tutor as my kids, in my own sessions. We have learned to ready pretty well by now, and I am moving now into conversational language. We're focusing on MSA, because we don't use the language "in real life" much, and either kid would likely end up with at least a quasi-academic use for it. I do have a pretty good grasp of a North African dialect, which is not much use outside North Africa. I learned it almost entirely without learning to really read, so learning to read has been a big deal for me.
All this to suggest bringing new baby and dh on board to maybe help instill a little more pride with the language/culture. One cool thing my kids have discovered is that languages are codes; dh and I do that thing grown-ups sometimes do, where we speak a common (non-English) language together in front of the kids and they have no idea what we're saying. Rude, maybe, but it always tweaks their motivation a little. What if dd1 were enlisted to help teach Dad and new sister as a point of family pride? Not to suggest that Dad's fluency is a goal, but maybe less of a feeling of either-or, to the exclusion of someone in your own little tribe? Would Dad be on board to get in on the fun?