Seriously...why bother?

So, my sister had a baby right after this past Thanksgiving. The only reason I've been able to see It since Its been born, Is because my parents came to town to come visit, as parents tend to do when they have a new grandchild.
The months leading up to her baby being born, and the months that have followed, I have kept trying to make plans with her. She always seems like, happy to do something with me and everyday leading up until "The Big Day" that we get to spend time with each other we keep going through all these different things we can do. And EVERY time, either the night before or the same day It's always "Oh I don't feel so good I think we have to reschedule" or "The baby doesn't feel good" or, my personal favorite, "We have to go to a baby shower today, I totally forgot until now"
...
This, right here, Is no joke. Every other weekend, If even that long In between, they have a baby shower to go to on her boyfriends family's side. Gee, no wonder she got pregnant, they must of not had a baby shower for that weekend!
Well, this past weekend me and my bf drove the 2 hours to Tucson to do some errands, and we were going to hang out with her and her baby and bf. They were excited about using us as an excuse to leave the baby shower early. So 30 minutes before we were set to meet each other, I call her to see If they're home yet, and she tells me "We can't leave, everyone got here late, we promise In a week or two we'll go down to your place."
...
Now I know this post will be a bit long, so by all means, you don't have to finish, I just really need to get this off my chest.
So at this point, im extremely pissed off. We had planned this for 2 weeks. We had canceled plans to do something with an In-town friend right before I called my sister. So we then got to call them back and make me look like a total douche for having a shitty sister. So needless to say, I didn't have the greatest of attitudes the whole time we hung out with them, which I apologize to them, but I'm sure they understand.
It's not like I live states away from her, It Is only a 2 hour drive If even, but that's far beyond the point. We always got along amazingly well for siblings. She's 4 years older then me but growing up we looked alot alike and always got mistaken for twins. After I graudated, I moved In with her and her then husband. Then when she left him, me and her moved Into an apartment together for about a year. It was really really fun, to bad she always had the boy she cheated on her husband with over there constantly.(Which Is why I left, his family has no problem giving him a free ride but I sure as hell am not)
So pretty much ever since I moved out, she keeps blowing us off last minute. I think of the hundreds of times we made plans, they only followed through on maybe 10 of them?
I'm not sure whats going on In her head, or whats going on In her home, or what she's trying to hide. But If this continues me and my bf are going to make It a point to randomly drive over there and stop by unexpectidly. It could very well be coincidence, but I'm tired of these "coincidences" happening last minute all the time.