There are a few ways this is explained, they could marry but not a sister or brother of the same womb. This was the rule.
1. This can either be because there was no law against this at this time. Therefore for them it was not illegal.
2. Each womb was a nation and this why they couldn't marry the ones who they shared a womb with.
This is a minor issue do we need the bible to explain this? Does it matter? Not really and we can use the bible but only to what we are allowed to within our own belief.