Re: Very disturbing
Very clearly, the customs and standards followed at that time were very different. This is yet another reason why islam has to be updated. Specifically:
Adoption is a very great service that makes both parties happier and better. It is very legal and in fact promoted by most countries with incentives. islam should make this change in itself.
By contemporary standards marrying daughter-in-law is incestual and from the fact that she divorced indicates it was so even then. I guess the divorce makes it not so bad but if people don't feel distasteful about this, there is something with them. Even marrying cousins, while not illegal or incestual, is certainly a bad practice as it leads to major health problems for immediate and/or future offsprings. Unfortunately this happens not infrequently and not just among muslims. I wish all religions will update themselves and discourage this.
Islam needs no update. As per your reasoning:
Islam doesn't forbid adoption. However, an adopted child will have different rights/duties than a biological child.
If Islam says it's ok to marry to an adopted child's ex spouse, it's ok, period. It doesn't matter what contemporary standards or anyone else has to say about it.