Unsure in New big sister at almost 20 years old

About 2 months ago I found out that my mother was pregnant. I dont really know how I should feel about it. I was kinda just okay with it but then later I began to really think. I was gonna be 21 years older then my sister. I mean thats a scary thought. I know alot of people go through the same thing. I think im more mad at the fact that shes pretty much lied about it. She kept saying that it was an accident, but they both knew good and well what they were doing. Not like she was taking a pill or anything. I also was on her phone one day and saw in the search bar how to get pregnant faster. And just so happens a few months later, bam, shes pregnant. She kept refusing to ever have kids again. She already has 3 and he has 1. Thats another thing that makes me not okay is the guy shes with, theyve been together for 2 years. Hes just a bad parent to his own kid. Never spends time with him, always avoids him, completly obsessed with my mother, he follows her around like a little puppy, he wont leave her alone to even hang out with his own kid. I just dont know, i will be moved out in a few months before the babies even born but I just feel like it wasnt the right time to even think about a kid. They already have a bunch on their plates. Why choose to bring a baby into that type of situation, i just dont understand