My mum became pregnant when myself and my elder brother were in our teens. Going forward two more years and she had another baby, as felt that child 3 needed a sibling of the same age. It all related to my father leaving so it was a bit of a complex situation.
So my mum was then a single parent to two under 3 and two teens, and relied on us heavily for help. She was going through a difficult divorce at the same time so things were tough.
But as soon as we elder siblings were able to, we were out the house. We were like the 2nd parent in the house, had to take responsibility for our younger siblings a whole lot whilst also doing important exams etc. I don't know about my db but I certainly felt that things changed a lot for us as teens, as we were no longer a priority. The babies came first every time, which of course they needed to, but it was quite a bitter pill to swallow when we were going through a tough time too.
The benefit though, is that they kept my mum young. She was all up on the latest fashion way before me, she gained a new friendship group with parents of children the same age, and I feel they did help her to recover from being in a dark place as she had just had to move on.
It took me a while longer to bond with them when they were no longer babies, then young children. I found them so annoying, demanding, taking up all of mums time, money, wearing her out etc etc.
We became much closer again when they reached their mid 20s I'd say, and now we are very close.
So OP, this decision absolutely will affect your teenagers, of course your situation is not the same as mine growing up, but I didn't fall totally in love with those babies right away. I know this sounds awful as I type it, but it is honestly how I felt back then. I needed my mum and I'd had 50% of her attention for 16 years, then overnight that 50% went to absolute minimal.
I just wanted to post from the other side, so you will have considered everything before you make your decision.
I wish you all the best going forward whatever you decide 💐