The pains of a close-knit family...

First of all, I want to apologize for going off topic..my intentions here were to document my eBay studies on mycoupons blog.

I have just been in so much emotional pain. I am 38 and have been married for 15 years. my husband and I have never been able to have children of our own...
I am the oldest of 4 children and there is a 13 yr difference between me and my youngest sister and 11 on the other.
Both of my sister became teenage mother's at a very young age... I have seen many things come and go in their and the children's lives. My husband and I have always been there for them...financially & emotionally. *FREE Babysitting* I have never, ever had a problem with that..that is all by my choice.
I just get so hurt when I get accused of being too close to my niece's and nephews. I hear in front of my back and behind "well you need your own kids".
I know, I know 99% of you are all parents and shaking your head at me!! Have I become too close to these children? What is the line of being an Aunt/Uncle? I give these children everything I possible can. I make sure that they are in football, have school clothes, etc. I try very hard and NEVER EVER tell my sisters how to raise their children. (I am being totally honest). Because my sisters needed help finishing school and getting on their own...the kids are very close to us as well. I don't want to be their mother...they love their Mother's very much. However, the youngest who is 2 1/2 now cries when they leave us and goes back to her Mother. (yikes) We had her for almost a year on and off due to school and work on my sisters part.( I know that has to hurt her) How do we break away and yet still be an active part in the children's lives? It becomes even more difficult that now the youngest child's mother is engaged and the fiance doesn't care for the closeness that my DH and niece have. He wants to be the children's father(if that's what my sister wants, I respect that)...he is now finding small excuses and making big deals and not letting the children spend time with us. I just don't know what to do...it pains me so badly all I do is cry. To see the kids cry when they see me but, I don't stay for more than a few minutes. If they do come with me, the future father makes comments whispering in the ears to the 10 year old.."Make sure they buy you lots of expensive stuff" !
The holiday's are coming and I don't know where to separate....I don't offer to babysit, I stopped going to football practices / games. To be Frank, I am starting to feel really used. I try to talk to my sister..it's just hopeless. You see, it's all because, I don't have any children of my own.(so, my sisters say) You can only begin to imagine the financial help I give her and never throw it in her face...I don't help her as a guilt trip to see the kids. I do it because, I love them and they live with my Mother and the the Future BIL has been out of work since Dec 2006. I feel like I can't have an intelligent conversation with them because, the STB BIL flies off the handle ! (won't go there) I know I have enabled both of them...how do I stop? If I do Stop, the Holiday's are coming...what if there is NO SANTA CLAUSE?

I am just at a loss...I am trying to plan a quick getaway. I do eBay full time and trying to figure our how to take my job with my and my DH on the road...
I just need a break from the stress...but, am I running away from my problems?
Thank you for taking the time and reading about my pain...
I just wonder are families that all live miles away from each other and only see each other on the Holiday's the way families are suppose to be?
for the most part...I could never see living anywhere else, I love having all of my family so close..just wonder is family supposed to be so painful?