My sister is going to leave her kids..?

My sister is 20. She has 2 kids. They are both babies. The oldest is not even 2 and the youngest is barely 1. Girl and boy. The girl has downs. They need her so much. Especially my niece. But my sister is...unstable. She's suicidal again, depressed and she honestly believes the best solution to this is abandoning her babies, giving the custody over to the father and my mother. It's killing me. They need HER. Their rightful mother. I hate my sisters for putting everyone, especially those babies, through this. She had them on purpose, cause she wanted to be loved unconditionally, but ever since she's had them she's neglected them, yelled at them, handed them off to anyone else. Now she's leaving them so she can "figure out things" and "find herself" again. I understand wanting to fulfill those things, I do. I'm only 18, very depressed myself and completely lost and want to run and figure things out. My plans were to graduate high school then travel for a while, just to get my life back on track, be happy for once. But I feel horrible doing that now, cause my sisters running too. But she has kids! And they need her! I will never forgive her for this! I cannot believe my mother is letting her run, instead of staying and face her responsibilities. I'm so angry and my heart breaks for these kids. They'll never know...their own mother. Although now I'm not sure they would want to. Life isn't fair and doesn't always go how you want...she's not leaving b/c she has to figure things out. She's leaving cause she misses being a teenager, partying, sleeping around ect. I just don't know how to get over this anger.