Confused, frustrated, and ambivalent?

I am not exactly sure what to write... I just know that I need to write. My sister is going to have another baby in October with her current boyfriend. They are also planning to get married at the end of April. This makes number three. The reason I don't know what to say is because I don't even know what to feel about it. I am angry, but not nearly as angry as I was about the others. I am frustrated, but part of me just thinks, "well, this is what she does." I love her and I love her kiddos, and I think her boyfriend is a swell guy, but I just don't understand how this keeps happening. I don't want to go into too much detail about her personal life, so I'm only going to share what she has shared with me... but it's just so confusing and frustrating trying to understand the situation and make sense of how I feel about it. I do not think her getting married is the best idea, mostly because of her history with her boyfriend. Like I said, he is a good guy, but I don't even think they know what they feel about each other at the moment. Like I said, I'm just confused. I don't know if I am just used to her having kids with every guy she meets (how sad, I know) or if I just don't have the energy to get worked up about this? The last two times this happened I was a wreck. I was obsessing about the future children, her relationship status, what my parents were going to do about it, how the kids were going to make it. I was making judgements and assumptions about her and letting my imagination run wild. This time, I think I am just not going to think about it. that might get a little tricky on her wedding day, but I don't know what to do anymore. I don't know how to express my disapproval without being hurtful or unloving. I don't know how to show her how I feel without her getting defensive and shutting down. I know God is going to work on me through this situation, and I am praying He will also work on her heart, and her boyfriend's heart. They need Him, just like I do, only they don't realize it yet. Please continue to pray for her and her growing family. I guess I should start embracing the fact that I am going to be an auntie again.