Myself (F21) and my partner (M22) are taking care of two kids (F12 and M14) for two months while their mom is in Canada for aggressive cancer treatment. I'm on nervous about it, need advice please!

Their mom is really not doing well, so her husband and her are doing a last ditch effort and going to a two month long treatment in Canada. The 14M kid is a really good kid, I'm not too worried about taking care of him. He is very solid and mentally stable for his age, but I definitely will be there for him anyway that I can. It's the 12F kid I'm worried about. I know she is having a really hard time with her mom's health, and is acting out in unhealthy ways.I care about the whole family a ton. I've known them for 4 years but my boyfriend has known then since he was born. I'm really worried about the 12F doing something drastic, like running away. Just see a lot of myself in her at that age, and I don't know how to properly help a kid who is struggling like that at all, other than just being there for her when she needs support and understanding. I still have to be the strict figure however (like making sure she cleans her room and does her chores, doesn't stay at friends houses too late etc).How do I be there for her and support her through this while still being a good parental like figure in taking care of her? via /r/Parenting http://bit.ly/2EyBXDz