I can disagree about same family languages. I think the jury is out because you get very confused exactly because of the similarities and can end up speaking "itagnol" or "portunol" etc which is the nightmare of Italian or Brazilian immigrants. It can be easier in grammar for sure.
Exactly, there is a point when proximity between languages means that instead of acquiring a new language, one of your existing languages morphs into the language you are trying to acquire.
I have seen great counter examples of people that could partition their mental spaces and isolate the languages properly.
I had one Spanish teacher in Brazil that could do this. I’m sure she had proper training.
I also had a Spanish friend and her mother was complaining that she was forgetting her home language because she was living in Brazil for some time. This is the most common thing to happen.
Interestingly, as she was telling me this after visiting Spain for about a month, I was having more trouble understanding her than usual, it can go both ways.
