Most jurisdictions do not allow sibling marriage and base that judgment on the long standing belief that consanguinity may lead to birth defects. Sweden allows marriage between half-siblingswith a government permit.
You can review the various laws at the related website.
Thiland
yes
Yes, it is legal for your sister to marry your husband's brother.
1 Biological, 2 Adopted, and Legal Guardian of his younger half brother.
If you marry him maybe.
When sisters and brothers marry it is called incest and is not a legal marriage in most parts of the world.
Assuming that by "brother-in-law" you are referring to your deceased's husband brother, yes, it is perfectly permissible.
A sister in law, by definition, is the sister of your wife. As long as you aren't still married to your wife, it would be legal. A sister-in-law can also be your brother's wife. You can marry her provided she is not still married to your brother.
No. A stepparent has no legal authority in regards to non biological children. He or she cannot give a stepchild permission to marry, leave the custodial parent or any other legal issue. If one biological parent has sole custody he or she can give the minor child permission to marry. If both biological parents share custodial rights, both must give permission for an underaged minor to marry.
no, Elizabeth is not a Frankenstein, merely a ward of the family. That's why its okay for Victor to marry her.
No. Even if they are not his biological parents they still have a parents rights which is more then a siblings rights.
As adopted the adoptive family becomes like your biological one so you can not marry the immediate family since that would legally be incest, but many states and countries allow marriage between cousins. If the adoption is reverted you can marry whoever you want.
They are not considered first cousins, unless your brother adopts them. If he did, they would be considered siblings and could not marry. Otherwise there is no legal relationship between them. They do not share a common ancestor.