In Rear Window, when Lisa sneaks into Thorwald's apartment and is caught by him, Jeff is simply watching from his Rear window. He does call the police but he and Stella do not try to go to the apartment and save Lisa. They do not even shout and call for help. Shouldn't this be the normal reaction? I agree that Jeff had a broken leg but Stella could have rushed to the apartment; after all, Lisa's life was at stake. I found that scene a little strange where Jeff does nothing other than wait for the Police. They were definitely not sure that Police would arrive so soon, so how could they just sit by their window and bite their nails doing actually nothing to save Lisa?
I'm not sure this is such a surprising reaction.
AS you already comment, Jeff had a broken leg and was thus fairly immobile. Stella was a care home nurse who is older than Jeff or Lisa. In the apartment, rushing Lisa, is Thorwald - a man that they both believe to be a murderer and who is much bigger than either of them. So consider these few points:
Firstly, what could they actually do? Jeff would be fairly useless and Stella has no weaponry or ability to take down Thorwald. This sounds like one of these situations where intervening would be extremely dangerous with little chance of success.
Secondly, from a more "human" point of view - Stella is Jeff's nurse. Would she want to risk her life by single handedly trying to save Lisa and stop Thorwald. I have many people I care about in my life, but there's not too many of them I would rush head on into a situation with a suspected murderer with.
Thirdly, remember this film is from 1954, a time when females were largely damsels in distress. It would have been out of place with the films of the time if Stella had managed to save the day and defeat Thorwald.
Finally, remember that this scene is the first time there is a risk of a true confrontation between them. Up until now, they have sent Thorwald notes and phoned him, but they haven't confronted him in person. Given all they know about him (or at least suspect) is that he's a murderer, their fear is again logical.
So I suppose in summary I'm arguing that there decision is quite rationale, since they think he's a murderer, they're not sure how they could stop him, especially given Jeff's injury, and they feel the police are the best option.