who should be made responsible for my sisters death? she was hit while in a crosswalk by a bus ?

The driver is definitely responsible (if they were negligent or careless, as hitting a pedestrian in a crosswalk would seem to be).

Whether the hospital is also responsible--the family or estate can maintain two separate law suits--depends on whether the care your sister received was adequate. If the blood clot should have been detected and treated, then the hospital would likely be guilty of malpractice; but if there was  no reason for them to look for or find a blood clot, then they probably are not liable. After all, people do sometimes die in, or just after leaving, hospitals when the care was first rate. So the hospital's liability will very much depend upon the circumstances and the quality of care your sister received.

I am a lawyer in CT and practice in this area of the law.the law will generally hold the driver liable to the extend that the doctor did not do anything to cause the death- this is commonly referred to as an intervening cause.  I suggest that a lawsuit is filed against both the doctor and the driver.  My firm actually handles these cases all the time.  Most of the time these case settle out of court.  I suggest that you hire a lawyer to consult with you on this.  The key issue is going to be whether your sister was at any fault for being hit.