This really is a Thor/Avengers crossover, but I wanted the focus of this to be on Sif and Odin. I think they have a really great relationship, and I bet Odin lets her get away with a lot
"Please, Allfather, allow us to journey to Midgard to aid Thor. He requires our aid in capturing Loki!"
Sif has always been a favorite to the King, Odin, as a potential bride to Thor, and was thus granted permission to walk with him hurriedly to the war room in the palace of Asgard.
"I and the Warriors Three feel we must give the Midgardians our battle advantage. They stood no chance to the Destroyer. How can we know with certainty that they are prepared to stop this traitor who controls the tesseract!"
"Silence Sif!" Odin bellowed to her.
"How dare you speak with such a lack of regard to my son!"
"I am sorry, I meant no disrespect." Sif bowed and they stopped walking.
"Allfather, I fear for how the Midgardians will treat Thor. They have proved themselves to be insolent and close-minded. How will they react when Thor brings his news, as I assure you his presence will not go unannounced." Odin, deep in thought as he often was, looked horribly conflicted.
"My Lady Sif, I am to apologize. I know what it is you are suggesting but at this time it simply is not possible. Without the bifrost there is no way to safely transport the four of you to Midgard and back. Thor was difficult enough, and the effects are starting to show on Asgard.
"I agree with you Sif, the Midgard people are not adequately prepared for such a threat, and yet we are powerless in stopping it." Odin sighed.
"My dear Sif, you have always been so loyal to Thor and to the throne. You have truly shown yourself worthy of your warrior's heart. You are needed on Midgard and I cannot aid you in getting there. My only suggestion would be to talk to Heimdall. Ask him to update you as well as the war council of Thor's actions as well as any decided plans of attack." Sif bowed and turned to walk away.
"Sif," She turned back and could see compassion in her King's unwounded eye.
"Should there be a need for you, no matter the cost for my health, I must order you to alarm me so I may allow you to aid the Midgardians to victory."
"My King," she stepped forward.
"Sif, should Midgard fall to Loki, next will be the fall of Asgard and the other realms. I am counting on you to make this call."
"Yes, my King." Sif bowed and rode to the end of the Rainbow Bridge where Heimdall stood. Odin turned to his war council and updated them with Sif's duty.
She may have been a woman, but out of the Warrior's Three, the Allfather could trust no other with the important decision that must be made.
Risk the safety of Asgard by summoning the magic to send her to Midgard, or to watch in vain as Midgard fell to the tesseract at the hands of Loki.
Never had the Allfather been so proud of the company is son kept. They would someday soon make a powerful court when Thor returns and balance is restored.