i am jealous of my SIL and for no reason..i just hate this feeling and i really don’t know y its developing in me..plz tell me how can i avoid it..
and plz don’t tell me how bad or selfish it is of me coz i already know that and thats y i want to know how to end it… 
Pray for her, it will make your heart and mind peaceful.
Re: how to overcome jealousy??
Well, I think that trying to figure out WHY you're jealous of her would be the first step in solving the problem. And I personally think that you do have an idea about why you're jealous of your SIL...but maybe for some reason you can't seem to articulate it.
How is she related to you? If, let's say, that she's married to your brother and you're jealous of the attention she's receiving from him...then you need to understand that the role of wife and sister are both special and can't be compared. If you're jealous that she's receiving attention from your parents....then considering the number of toxic in-laws stories we have here...it's a good thing that people are trying to make her feel comfortable as part of the family.
If she's not related so directly to you....then are you jealous because you think that she has something you don't? Such as attention from in-laws/spouse....kids....or some other quality such as looks, intelligence, etc?
Sorry, I don't believe that you're completely clueless about why you feel this way. Jealousy is a strong emotion...and usually people have an idea about what's causing it.
If you have a pretty decent relationship with her...then you need to keep those feelings in check. They go against the spirit of Islam and I'm sure you already know that our culture seems to be rampant with toxic in-laws stories. Do you want to be yet another negative example...or do you want to make the effort to rise above and set a positive example for others? It's up to you. Understand that our body language reflects our emotions...even when we don't verbalize them. And often times those negative emotions will manifest themselves...and you'll risk hurting your relationship with her and with other family members...you'll be hurting your image and it will be sinful.
If you're jealous of some quality that she has that is not even in her control (something silly like her skin color, etc)...then it's pointless to get so worked up. If it's something that you can develop through some sincere effort of your own....then do so. Sometimes we think that a person acquired a particular thing with ease...when it actually required work. Life is full of highs and lows for everyone...including your SIL. And trying making positive duas for her...that might relieve some of your tension and ask Allah to bless you with whatever it is that you feel is lacking in your life. If your SIL is instigating feelings of jealousy by showing off....then it could be a reflection of her insecurities. Or perhaps what you perceive as showing off is nothing but her sharing her happiness others. You both are unique individuals and you have your share of wonderful qualities as well.
I've listed several possibilities but again....I think it would help for you to admit to yourself why it is that you feel jealous of her. You must have an idea. I understand it can be easier said than done.....but there's a hadith which says that a person is not a Muslim unless he wishes for his brother what he wishes for himself.....and unfortunately jealousy just fills a person's heart will ill feelings toward another. Maybe if you remind yourself that it has the potential to hurt you iman...it could help the situation. Feeling jealous does not make you a bad person. It's a natural emtion and most people have to make a CONSCIOUS effort to think about things in a more positive light. It's not something that just disappears magically.
Re: how to overcome jealousy??
its not your fault..........it comes naturally to desi women..:(