Berkshire reported selling a small number of shares

Berkshire didn't sell any Liberty Sirius shares. You are probably thinking of a small sale recently of Liberty Live Nation shares, which is a different tracking stock.

U.S. Securities and Exchange Commission: SEC FORM 4

SIRI just had another large short squeeze. It is not currently possible to short SIRI to capture the arbitrage spread with LibertySirius. Some market participants exiting their SIRI short in the squeeze may have been long LibertySirius in a spread trade trying to capture the arb. Those exiting SIRI shorts would be buying SIRI and selling their offsetting LSXMA/LSXMK shares to fully exit - further increasing the spread.

I don't know what price newSIRI will trade at after the deal closes. With LibertySirius at a huge discount to SIRI shares, there should be no natural buyers of SIRI shares other than those covering a short position / option position, or traders just trying to participate in the price movement from the squeeze. No fundamental investor would choose to buy SIRI shares as a long term investment (they would buy LibertySirius instead).