Is it helpful to tell a university that I have admission and scholarship from another university? I have got an admission letter with full scholarship (i.e. covering tuition fees, living expenses, etc.) from a top university X (ranking under 50) but I want to take care of my grandmother who is not from my birth-country so I want to continue my Master program at that country B; which I don't have its citizenship; so I will be a foreign student and the number of full scholarships (even partial ones for covering tuition fees) at country B are very limited for international students. Also, the university Y at country B has ranking between 2000-2500. 

My question is : Is it rude to telling (and emailing) them my admission letter that I have from university X to university Y? And will it be helpful to enhance chance of getting scholarship or quite the reverse? 

Thanks!
Yes, but with caution.

The basic premise is, if you're an outstanding applicant, it's conceivable the admissions people at the lower-ranked institute think it's pointless to accept you because you're going to decline the offer anyway. If you've been accepted at Harvard but rejected at Saint Mary's, chances are that happened. In that case you can send them your Harvard acceptance letter and tell them why you really would rather attend Saint Mary's. Similarly, if Saint Mary's accepts you but doesn't give you much funding, then sending them your Harvard acceptance letter with full scholarship is likely to make Saint Mary's add more dollars to your application as well.

Before Saint Mary's makes a decision however, I would not send in the Harvard acceptance letter, because it's irrelevant.