I think we might be over complicating Lev. 18:18. Read simply, it says don’t have sex with sisters at the same time if it is vexing to them.
Now that is a different interpretation, and it brings up the point that there is
variance in the translations; I suspect that the Hebrew grammar here is unclear.
Once could almost take that interpretation from the KJV...
Neither shalt thou take a wife to her sister, to vex her, to uncover her nakedness, beside the other in her life time.
Especially since it says "beside the other". Except that it says "neither shalt though take a wife to her sister". No point in saying 'don't marry' if all you really need to do is not have a threesome. And no point in mentioning "in her life time" if it was communicating 'no threesomes'. So that doesn't make sense.
The NIV sounds like you could take her sister as wife, so long as you were celebate with her...
Do not take your wife's sister as a rival wife and have sexual relations with her while your wife is living.
But that kind of defeats the point, so it doesn't make sense.
One thought for thought translation (marked to children) puts it thus...
Do not take your wife's sister as one of your wives, as long as your wife is living.
They entirely leave out the two sections which are often put in comma's; indicating they are explanatory for 'do not marry' rather than pat of the command. That might be true, but it's a poor proof.
But the Septuigent is interesting...
Thou shalt not take a wife in addition to her sister, as a rival, to uncover her nakedness in opposition to her, while she is yet living.
In all they have 'while living', so clearly one can marry a sister of your deceased wife. Why would that matter? Because the rival and nakedness could not apply.
That is an argument in favor of being able to marry so long as your motives were pure. But it's an even stronger argument that it has nothing to do with motive. If it is to be interpreted "do not marry with impure motives", then there is no point in mentioning 'while alive'. But if the problem is bringing in a woman who would inherantly be a rival (as sisters are won't to be) then it would be necessary to mention the living so people would be free to marry a sister after the first dies. Now, twins likely wouldn't be rivals. That could be why they also mention nakedness. For some reason having sex with two sisters (at once or seperately) is a problem.
Alternatively though, one could view "to uncover her nakedness in opposition to her" as an explanation of how that rivalry is provoked. Which we can see in the example of Jacob's wives. For really close twins, that might not be a problem. The only issue with this line of argument is: this issue can be a problem with any wives, regardless of relation.
And that points to this being a close relation (v17) problem.
But maybe the problem here is in reading this legally and theologically. Maybe Jolene has it right, this is talking about situations; that's how most people think. v17 is talking of the step-daughter step-granddaughter situation (since sometimes grandparents end up caring for grandkids) and v18 is talking about the marrying her younger sister situation.
From that perspective, marrying twins at the same time is a different situation (and one often without vexation).
And that makes sense in the historical context; since most of the time daughters would be married off as soon as they came of age. The usual time a sister would be available is when the next one came of age (or if one got divorced / widowed).