HI Ashlea,

Although my brothers aren't as much older as you will be with your sisters, we have a pretty close relationship. We are 10 and 9 yrs different in age (they are older thatn me) and I have a great relationship with them, my second brother in particular. When we were little mum used to get them to look after me alot while she did things like cook, house work etc...which I think helped them bond with me. They have always been very protective of me and very loving. Now that we are older, my eldest brother has become less close to me, though I think he might be a bit jealous of my DH to be honest and feels a little threatened by our relationship, like he feels replaced a bit, IYKWIM? But my middle brother and I are still very close to this day and he now sees me as more of his equal and treats me as such, no longer teasing the cr@p out of me so much!!
At my work there is a family in the same situation as you will be, with the mum having a baby with her daughter. The kids are aunt and nephew and are 3 and 2 yrs old, and couldn't be closer if they were bother and sister. The two sisters one 23 and one now 3 are also very close. There are also four other girls in the family, 20,18,16 and 5. Though they all still live in the same house and are as sisters anyway. The little one stays over at her sisters house regularly with her 5yr sister and they all have such a lovely time together. Despite the age differences, they are all really close with each other.
I think if you make the effort to be close to your sisters, then it will happen.