Was Tamar’s offer to marry Amnon sincere and plausible?

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In 2 Sam 13, Tamar attempts to persuade Amnon not to rape her, entreating him (v. 12):

Now therefore, please speak to the king, for he will not withhold me from you.

My impression was that Tamar was Amnon’s half-sister, assuming her parents are to be equated with Absalom’s (v. 1). This relationship would be considered incest according to Lev 18:9, etc., so I’m wondering whether statement was meant as a plausible offer. I recently heard the claim that they were actually step-siblings, which would make such a marriage legal. This seems to detract from the nebālāh / nabal (ESV: “outrageous thing / outrageous fool”) of vv. 12-13, but perhaps rape itself qualifies.*

• What was the family relationship between Amnon and Tamar?
• Should we expect the Mosaic laws on incest to have been observed within the royal family at this time?
• Did Tamar actually expect that David would have been willing to grant such a marriage?

_{* As in the Dinah story, widely cited as a parallel. It’s hard to believe that such a thing was “not done in Israel“, but perhaps this is just a manner of speaking. Her claim that “this wrong in sending me away is greater than the other” also seems to indicate that she thinks marriage is possible, indeed preferable.}