Aerys and Rhaella were siblings, but their parents, grandparents, great grandparents and great-great grandparents were not. Because of this, yeah, we can pretty much ignore the Targ family history of incest genetics wise, but Aerys and Rhaella were siblings.

Under normal circumstances, genetic similarity between siblings generally ranges between 45 & 55%, but 35-65% is reasonably possible, and less/more is theoretically possible, though incredibly unlikely.

If Rhaella and Aerys were lucky, and around 35% genetically identical, it's very likely that Rhaegar and Daenerys were less than 75% genetically identical (which is, I think, the number that that post came up with).

If they did share 75% of their DNA, chances would be that Jon would be around 50% genetically identical to Dany, which would be pretty much the same as first generation incest, and not be all that risky.

It is, however, totally within the realms of possibility that Rhaegar and Daenerys were 50% genetically identical or less, with some luck, which could actually result in Jon and Daenerys sharing relatively little DNA.

Now, this is a work of fiction. If GRRM says that's what happened, so long as it's possible, there is no sense in trying to argue otherwise. It's unlikely, but unlikely means that it can happen. Given how different Jon and Dany are, both in the show and the books, I think it's a pretty safe assumption that that is what happened.

Bare in mind too, that there is an active God in this world, and that God might have skewed things in their favour as Jon and Daenerys are pretty important to him, so far as we can tell.

So yeah, genetically speaking, there's probably not very much risk with Jon and Dany having kids. Most likely scenario is that they are as related as normal siblings, which is actually not that risky, and just a bit of luck makes 25% similarity reasonably likely (which makes the increased chances of problems completely negligible).

This really brings it all down to a discussion about what makes incest gross, I guess, but Jon and Dany probably don't have any genetic problem, and they only met as adults, so I'm not really sure I can come up with a reason that they shouldn't be together.