When i learn languages that are from the same family as my native language, I try to see them and I treat them as totally different languages in order not to get comfortable and start mixing them as if it was okay and normal. 

I am native speaker of portuguese and when I started learning spanish, it was painful. Painful because it was so hard not to want to mix vocabulary, pronounce the words like in my mother tongue, make sentences with the logic of my mother tongue, use the grammar as if I were using my mother tongue's one. Spanish was the language that I put most effort in in order to learn comparing to Russian, German, English, Slovak, Turkish and now Arabic. 

Also, once I wanted and tried to learn italian, it was a DISASTER because everytime I would open my mouth to read or speak italian, I was reading it totally as if I would in spanish and I remember the few italian sessions that I bought and the teacher would ask me:

Teacher: "Chi è quel ragazzo?" 

I would answer: 

Me: "Oh si, este muchacho es mi hermano" 

I would hear the word "ragazzo" (which means boy), would passively understand it but once I would try to reproduce it with my mouth, "muchacho" which is spanish would come out of it. Terrible! I ended up giving up on italian. Now I am learning French because I am soon travelling to Morocco and Tunisia (where french is widely spoken) and even though it's a language from the same family as my mother tongue, it is actually very different, especially in pronunciation so I don't mix it a lot but still I don't get comfortable and underestimate it, I treat it as if it were arabic to me. 

As a portuguese teacher here on italki, 97% of my students who already speak other romance languages constantly mix vocabulary, grammar logic and pronunciation and I don't find that okay. :)