This seems to be a common phenomena among twins. Further marrying sisters is a historically common form of polygyny. I'm not sure how to rectify that with Lev 18:18 yet.
Since God who is perfect in all his ways showed Himself as concurrently (during their lifetime) married to two sisters, and since I can't find any sense wherein He somehow set things up so as to vex one with the other, then I'm having to conclude God got it right (of course!). My read of it is that He accomplished marital union with two sisters without the taking of them being the cause of vexation. Any disagreement they had between them was not stemming from some faulty act on His part.
In contrast to taking one to vex the other, I've heard of twin sisters who were forced apart by monogamy-only marriage, and for the rest of their days they had a form of vexation due to an extreme sense of loss. They were conceived together, born together, grew up together and were then forced apart for the rest of their days on earth. Talk about being vexed!
Imagine if God had applied monogamy-only to the two sisters He took and told one of them "you have to go away, you can't be mine because only one is allowed". That would have been awful and definitely vexing! No, in Leviticus 18:18 we are told what is wrong to do with sisters, then we have God's example of perfection with sisters that everyone has to concede is right. I take my cue from His example since it can't be improved upon, plus, I take the wisdom of Leviticus 18:18 and make sure that in my life I won't do that. Set before us in the Scriptures are both God's example of getting it right and Leviticus 18:18 telling a circumstance that would be wrong; neither contradicts the other. I don't believe God says one thing but then acts the opposite. People do that, but not God.
Have a look at expressions on the faces of those two ladies in the link - if the guy told either one of them to go away because he can only have one, I'm pretty sure that would have been the cause of rather serious vexation which neither lady would get over for the rest of her life. Such words from him would undoubtedly have caused vexation. Taking two sisters as God did just strikes me as a rather good example that cannot be improved upon, but it definitely is an example that can be followed. Who would say "don't follow His example"?