First thing is, I genuinely believe that it is the extreme minority of people who set out to be deliberately mean, or selfish or unkind. Those who are deliberately unkind - who know they are being cruel for no reason, and with no regard for how their behaviour affects another person - are very damaged people. The majority of the rest of us are just trying to do the best we can with the resources and insights that we have at any given moment in time. I have spent a lot of time thinking about this, as I grew up with an alcoholic mother who was abusive and neglectful. And even though she did some terrible things to me, I honestly believe that she was doing the best she could, and wanted to be a good mother.
I suspect that your sister falls into that category of people who wants to be good, but for whatever reason (whether it be perception or circumstance) is falling short.
I also think that you stand a better chance of altering people's behaviour if you try to understand their motivations rather than condemn them. Why is she acting as she is, and why does she think it's acceptable? Do you really believe that it's because she's just a bad person? If so, how did she get to be that way? Do you think she's just a bad spirit, who was born into the body of your mother's offspring to torment her? She probably feels (rightly or wrongly) that she is justified in her behaviour, maybe even that it is good (? people sometimes believe the strangest things)
With the niece, the same factors apply. Teenagers tend to be more selfish than adults (although this is not always the case) - but I do not think this is because they are innately any worse in spirit. It is just this understanding of other people's viewpoints, and the impact that your actions have on others, sometimes takes some time to develop. I didn't start to develop this until I was in my thirties, so I behaved like a selfish teenager for most of my twenties
Another thing to consider is that, as the saying goes, "Whatever you resist persists". This is a really difficult balance to get right, and one person's "just trying to be helpful" is another person's "unwelcome interference and condemnation". I know you want to see these people do the right things, but you have to consider how likely your actions are to have the desired effect. If people feel like they are being nagged or condemned then (again, rightly or wrongly) they will turn off and stop listening to you. it takes a great deal of empathy to see the viewpoint of someone who you feel is behaving badly, but the rewards can be enormous, and ultimately lead you much closer to your goal.
i'm not sure how much else useful I can say at a distance. I think it might be better to try to influence their behaviour through meditation and sending love, and thoughts of correct action, rather than direct words.
I hope this helps some.