I can't imagine any natural pressures that would make this happen. In fact you're more likely to have them either:
(A): Lose the legs entirely, elongate the spine, and have fins form by spines of keratin(hair) coming out of the elongated body and having webbed skin between them. Think aquatic lamia/snakeperson, or more aptly for mammals like us, some sort of dolphin-like lower body, though like the dolphin you'll find that we may become more streamlined as well, probably losing much of what we'd call a humanoid body shape for our upper, or in this case frontal, body.
(B): Have the feet become what many swimmers put on their feet anyway. Fins. The bones in the feet would become extended and turn into cartilage instead of purely hard bone, while the extended toes would get webbing, creating the fins that an underwater humanoid would make use of but suffice to say this would make it so that they can only traverse on land on their hands and knees, assuming the hands haven't turned into fins as well at which point they'll probably need to use their elbows.
What you're seeking can only be done with functional-purpose surgery, where the body is artificially altered to serve a purpose, but this seems excessive compared to simply slapping on a pair of synthetic fins.
As per your example disorder, sirenomelia does do basically more or less what you're envisioning, with regards to the fusing of the legs specifically, but it is wholly detrimental and I severely doubt any kind of functional species could come from it unless, once again, the condition is altered artificially to make it less detrimental and more functional.
Actually, now that I've thought about it...
It is somewhat possible for your mermaids specifically to come about naturally, but it is a rather convoluted way to go about doing this. It is a three stage process, laid out as so:
Stage 1: Humans regain their tail. I don't what kind of pressures we would need for us to regain our tail, but whatever it involves it must prioritize reproduction with families who tend to give birth to children with vestigial tails that can then through sexual selection be nurtured back into a functioning spinal structure. All this is a relatively natural way that can be sped up artificially, assuming laws and such on human genetic modification is ignored.
Stage 2: Humans with tails become more aquatic, gaining webbed hands and feet, probably hairless skin(or the way of the otter/seal, insulating and aquadynamic fur, depending on the climate and various other pressures), and a more aquatic variant of their tail that will either be vertically flatter or horizontally flatter. This may be forced with melting ice caps making the sea level rise and making humans more and more likely to spend time in water, while somewhat increasing the likelihood of consuming fish due to there essentially being more ocean as the water levels fill up land spaces that used to be ocean once more. An unforced development is unlikely, but can still happen with some group of tail-having humans preferring to spend a lot of time in the water and their survival depended on catching fish, aka swimming, if fishing rods and the like somehow were forgotten about.
Stage 3: The more aquatic humans, which you'd be hard-pressed to call humans at this point, merfolk would be more apt, spend much more time underwater and in fact have become somewhat dependent on the water for their survival. Their tails now have fin-like structures, as well as their hands and feet having become elongated and a little fin-like as well with more developed webbing but due to the merfolk probably still making use of tools the hands haven't become entirely fin-like. It is here where your specific mermaids come into being. There would occasionally be a merfolk who gives birth to a baby with sirenomelia, causing fused legs, and most importantly, a fused tail whose spine would do down to the ankles or further most likely. The fins of the legs would most probably be gone, lost in the fusion, with the occasional siren(what they'd probably be called) having vestigial fins coming from the mass of their lower/back body like those tiny fins of a shark and might still help them to swim in some cases. Due to the tail being most likely longer than the legs at this point in their evolution, sirens would still have a relatively functional tail beyond where their footfins would be fused, who would not be too badly afffected by their sirenomelia, and would as such create the mermaids that you're looking for, perhaps with some inheritance factor to the condition, causing sirens to generally make more sirens and as such create a subspecies of merfolk who for all intents and purposes are basically mermaids. This could also make for some interesting cultural, social, or societal interaction between the normal merfolk and the sirens, which if you choose to go that path I'll leave the details up to you.