I'm sorry this has happened to you, too. I can relate because my younger sister, who is only a year younger than me, has three children under age 8 and will have her fourth child this summer - and she readily admits the fourth child wasn't planned.
Seven years ago, before her first two children - twin boys - were born, my sister was lots of fun. She and her husband, and DH and I, went out whenever DH and I visited them - and that was several times a year because DH's parents live in the same city where my sister lives.
Now, if the four of us go out, it has to be someplace where the kids can come along, since my sister doesn't like to leave them with babysitters other than family. That means no more going out for nice dinners or drinks or other adult-oriented entertainment. Most of the time, when DH and I visit my sister, we spend much of our time playing with the kids. I love my niece and nephews, and I'm sure I'll love the new baby, too, but having to entertain them all the time gets boring.
I can't even go out alone with my sister and while the kids stay with DH and my sister's husband, because Sis doesn't want to leave the kids with their own father and uncle (and my DH is great with kids; he's a former teacher). I guess she thinks that taking care of kids should be "women's work" most of the time. So I can't go shopping with her or do girl things with her anymore.
I realized how much I miss how it used to be between my sister and me this weekend, when Sis came to visit our aging parents alone, for the first time in more than eight years. My mother was in the hospital recently for surgery, and she didn't feel up to a visit with the grandchildren, so for once my sister left her children alone with their father for a weekend and drove the 300 miles to spend the weekend with my parents and help my mom and dad. My DH and I live only an hour away from parents, so we came over to help, too. And for the first time in years, I was able to have a conversation with my sister without her being distracted by the children or the children interrupting. Granted, we weren't able to go out and have fun - our conversations took place while we were doing housework for my mom or cooking a meal - but it was a nice change! Too bad she'll soon be even more distracted with the new baby.