Advise would be nice!!!!

Whisper

So my sister in law had her 2nd daughter 7 months ago. My husband and I have had her off and on since she was born due to her mom having drug problems. My husband and I have two boys of our own ages 5 and 6 years old. We now have temporary custody of my niece which is hard because we have nothing for her. Her mom gave up her 1st daughter when she was around the same age as this one and she hasn't seen her in years... so I absolutely terrified to think that this baby is going to be mine? Should I be thinking this way or should I be more positive thinking. She is supposed to go get help in rehab but we have had kinzie for 5 days now and she has yet to even mention anything about rehab to us she's called once to let us know she needed money cause her car broke down she never once asked how her baby was doing. Should I be thinking she is going to be mine or should I be thinking oh this is my niece and I'm just baby sitting her for a really long time haha please mine and my husband emotions are all over the place not to mention we both have full time jobs! So everything is hectic right now. And our boys are starting to show a jealous side and I'm worried that they are upset and feel as if we are trying to replace them. Because she 7 months old and can barley crawl she has no schedule and she can't even hold her own bottle.