I completely understand trauma from having step kids
It's definitely a thing.
What strikes me about your situation is the timing.
Teen agers are awful. Sorry teen agers! And your SDs have a mom and step dad who seem like chronic teen agers themselves. I mean, I know teens have wonderful qualities and it's an amazing time in their lives and there are definitely ups. But wow. What a challenging time for parents when teens start to gain momentum!
I remember reading somewhere that the teen brain starts to rewire so it can learn how to learn socially from peers (versus from adults). The quasi rejection of parents and adult figures is built into the process for how kids become adults. But so is the attachment to parents, which probably explains why our kids come to us for stuff while believing we know nothing.
And apparently the definition of popularity changes for kids as they go from elementary to middle to high school ages. In high school, popularity is dictated by the willingness to test taboos and limits. This kind of tapers off when teens become young adults and settle back into what feels like themselves.
You have a very challenging step parenting dynamic and these teen years are probably going to get weirder and harder given mom and stepdad dynamics.
It's a super personal decision to have a baby, obviously and I genuinely have no thought one way or the other -- you're being so wise to consider your own capacity so carefully. I just think this period now as the girls go through their teens is kind of like the month of February. It's often the coldest, the darkest, you can't remember last summer and spring seems like a long way away...
It's almost like you have to take into consideration that this too shall pass so you have enough space to make a decision that is genuinely for
you.
That, and having a baby is such a major reset of unknown proportions.
I was a step mom to n/BPDx's teen age son when I had my son, and it created a dynamic I could never have anticipated. My step son had to adjust to no longer being the center of attention and honestly it was the best thing that happened to us. We bonded over focusing on this tiny little person and it changed the dynamic into something way more real and frankly, pragmatic.
That's the kind of stuff you can't really plan for because it takes its own path into places there are beyond planning for.