Need some advice...what to do next...?

My 18 almost 19 year old sister kate has had some tough times recently, and she ran away to her boyfreinds leaving her daughter, my 2 year old neice with us my 2 year old neice is named flower by the way we have a great relashionship =) she loves being with me and is always happy to see me and always wants me to cuddle and play with her, so its really sweet =), i have to care for my 4 year old sister her name is kayla since my parents are always busy i also have a brilliant relashionship with kayla, who always wants cuddles and tells me im the bets big brother and tells me she loves meee so much and always wants to play, but since i have been looking after two while my parents have been busy its been hard looking after them both but i think i done well =), today my 18 year old sister came back, kayla and flower were both napping, but flower woke up when my sister came into the room she was, so my sister told me she wanted to see flower, so i let them have time alone, after about 10 minuites i heard flower scream so i ran down and she had a big red saw mark across her face, so i knew what happend and my 18 year old sister ran out crying she got her coat on and left before she admitted she did hit flower, so right now im with my 4 year old sister and my neice, i dont know what to do to make my 2 year old neice feel better..... the weekends coming so could i take them out? and where would be good? if you mention mummy my 2 year old neice will just scream no! and start to cry, i need some advice, my 4 year old sister kaylas a bit confused so i told her not to worry, but my 2 year old neice flower is taking this hard, will the mark go away.... im sure it will but just checking, and how can i make flower feel better? i have a great relashionship with them as i mentioned so they trust me and being with me makes them happy right now, but i wnat to cheer them up, so any advice?