1) I want to ask that some of my cousins used to talk to their na mehram friends and I am trying to stop them from doing this but the fact is that they got angry and even their parents got angry with this. What should I do for them to not let them talk to namehrams as i know it will harm them in future.

2) One of my cousin don't want to do perfect hijab and we other cousins are afraid that if we do tell her to do proper hijab, she might not like and even her parents will not like?

In both the situation what can i do? Should i leave them and not say anything or should I tell them to proper their acts?

1) You can not do any thing more than advising them, and telling them them that what they are doing is haram.
Ask them if they agree that the same thing happen with their sisters.

2) If there is probability that she listen to you, provided that you advise her in nice way, then, you have to do so. While if this probability does not exist, then you do not have to.