Half siblings isn't the worst case of incest that could be, especially when the daddy's ostensibly blank slate as far as lineages go. Sets a slippery precedent though.
As above incest is a slippery slope, but in the world of divine beings and magic, genetic mutations due to incestuous couplings may be non-existent thing. Least this is something I choose to believe until the author adds it to the setting.
Having half-siblings myself, the thought of marrying them is just gross.
Same with marrying 1st cousins, but since knowing my family tree, the process itself doesn't seem bad. As long as it aligns with religion and custom, so be it.
That being said, it means depending how they are brought up, those two kids might not be against it. First iteration might not go wrong, but it should be avoided to turning into culture.
Marrying them together won't solve the main issue: if anything, it would further solidify Abraham's position.
You would avoid conflicts by giving him the kingdom without a fight: sure, it's a solution, but are you sure it's the solution you want?