if the WB distribution was taxable then your basis for Sina would not change from the original
if it was a tax-free distribution then your cost/tax basis is supposed to be allocated. there is no one absolutely correct method
maybe WB or Sina sent the shareholders info on tax basis allocation on each distribution
if not then here are two methods
1st distribution
fair market value at the end of the day of the distribution for WB (a)
same for Sina (b)
basis allocated to WB original tax basis of SINA times (a) divided by the sum of (a) + (b)
basis allocated to Sina - total original basis less above or tax basis of SINA times (b) divided by the sum of (a) + (b) (They should be the same)
do the same for the second distribution but take into a/c that your original tax basis for SINA was reduced by the tax basis allocated to WB on the first distribution
another method: use the average of the opening and closing prices for the two stocks on the distribution dates.
announcement about the distributions
https://www.investors.com/news/technology/chinas-sina-announces-distribution-of-weibo-shares/
https://www.prnewswire.com/news-releases/sina-announces-distribution-of-weibo-shares-300464527.html
webpage to get WB prices
https://finance.yahoo.com/quote/WB/history?period1=1451606400&period2=1483142400&interval=1d&filter=...
could not find a webpage for SINA however, it did trade on the NASDAQ
