Do you actually want a child of your own? Or are you simply enamoured with the compliments your DH gave you (and who wouldn't be? I love it when my DH says things like that haha)

As for it being a 'bizarre' dynamic because skids have a half sibling on their mums side already, I wouldn't worry about that.  If you & DH decide you want a baby, that's all that matters

Are you excited by the thought of being a mother or just by the thought that you'd be a better mother than BM?

Also, does your DH actually want kids, or does he just want a do-over with a new woman since you won't be replacement mom to the ones he already had? As in, he doesn't *really* want to raise more kids. He just wants his name attached to good kids, and since he knows you'll raise good kids, he's wanting to knock you up.

I don't want to take away your buzz. I COMPLETELY understand the feelings you have from being complemented in the way you were. Hell, there is a part of me that giggles solely because DH getting a vas reversal is a big middle finger to BM's scheming (backstory: DH and BM agreed to him having a vasectomy because their marriage was on the rocks and they didn't need more kids; DH does it and BM serves him less than a week after; I have zero doubts she plotted that so that he couldn't have more kids and to make her beholden to him forever).

BUT, don't do it unless you really WANT motherhood and your DH really WANTS fatherhood. Don't let the sole reason be because you want to prove something to BM. If that is a bit of icing on the step hell cake, then great! But don't make a cake just for the icing.

Hmmm I feel like we’re in the same boat. Although I know I want a kid and my SO is unsure. He’s warming to the thought and knows he’ll be an older dad (he’s 41 now) but only has one son, which is SS9. I know the age difference may be odd for SS, but I grew up with two older half siblings and I actually am closer to my oldest sibling even though we’re 10 years apart in age. The half siblings thing isn’t even a thing. And I hate to be this person, but I KNOW I’d be a better mom than SS’s BM, cus ya know...I didn’t leave my kid and move half way across the country and hand him off to dad. Listen to yourself, what do you actually want and do you desire being someone’s mom? Do you have all that love to give and are you up to the challenge and learning experience? Or did you just enjoy knowing that your DH knows how amazing you are and vocalized it? also who cares how old DH is or looks, he’s your partner, if you two want something, get it 45 is hardly old. 

Ohhhh, I totally get this OP! I am a much better mom to my SDs than BM is and DH is always so appreciative and complimentary about it and every now and then we have the "wouldn't it be nice if..." conversation or I get a bee in my bonnet about how "he got to do this" (meaning having babies) with BM and we don't have "anything together" and the skids are with us full time now anyway so being childless didn't turn out to mean being child free and I start thinking, "Yes, we should have a baby!"

I can't speak for you and your DH, but for me and mine... no. No, we absolutely should not. LOL.

When I really sit with my own thought process, I know it's motivated by a weird belief that sits at the junction of fact (DH had a family with BM and not me) and a socially constructed belief system (bio families are real families and real families are better than step-families) that leads to an inaccurate conclusion (DH and I having a baby will make us a real family, thus making us better). That's really simplified, but the thought pattern looks something like that. And the logic is full of holes.

DH did have kids with BM and he loves his daughters. He would also hop in a time machine and redo his decision to marry and mate with her in a heartbeat. BM destroyed a huge part of his life and his business, during and after the marriage. She cheated on him, she used him, she hurt him, and she tried to turn the kids against him. As such, DH does not think being a bio-family or even a bio-mom means jack all to a marriage, a life, or one's happiness. So my idealization that a "bio is better" isn't even something my DH agrees with. Finally, the stress of child rearing on a marriage is completely real. My idea that "having a baby together will make us a real and better family" is not at all a given, as evidenced by they fact that having kids did not make anything better between my DH and BM and actually made things worse.

I don't know what my logic would sound like if I really truly did want a child with my DH but it wouldn't be this. This train of thought is a questionable method I've concocted to define my identity inside a family that was formed before I arrived and gain a standing of greater significance to the outside world, since most people are dismissive of the "stepmom" title.

I think you'll know shortly which line of reasoning you're using, but I found it hard to sort through it when DH and I first started seriously discussing the idea.