It sounds like your niece has a diagnosed speech disorder? If so, your sister may be having a horrible time dealing with a world that truly wouldn't understand. I know--btdt. Sometimes, we become hermits just because being out in the world is hurtful and even MORE depressing. When confronted with family that don't understand or show concern for how we live, it's easy to get defensive.
Parents of SN children often homeschool because truly, the public schools have a hard time meeting the needs of some of these kids. But moreso because the social and emotional backlash to our kids really isn't worth it and could cause even more stress and depression--not just to the parent, but now also to the child.
For parents of SN kids who are over 5yo, the support is close to nothing. And if you're homeschooling, it's even worse because the schools won't even help. So you are really on an island. Now let's add a society that lacks in social graces or etiquette (aka staring and rude remarks made by people who don't understand and really have no business making comment). And finally, family who don't live our lives day to day... and now you're left defending and explaining to people who are supposed to just love you and help you. And all this time you may still be mourning the loss for the life your child could have; and wondering what other thing you could possibly try to make it better--with no help. Sometimes, not even from your spouse.
Add to it a community of medical and therapeutic practitioners that often push you to doing things that have not been tested and have no promise of a result, but carry serious warnings. And if you refuse, you are left to fend for yourself. Worse yet, if you have a child whose condition is not well understood even by the "specialists".
Who wouldn't be depressed?
If you are concerned for the child and her future, ask your sister what you can do for her. Be there for her. Understand what they are dealing with. Research the child's condition. Find out what the options are. Find out which of these your sister has tried or why she decided not to--and try to respect that these are hard decisions. It's easy to say what you would do if it were you; but often, you really don't know if that's what you'd do until you are there.