Hi @Ant7,
as long as you are there and you are trying to help, then I think you are doing the right thing. The worst thing (for me, anyway) is when people withdraw and we feel alone with our worries. It's so good to have loved ones around us, supporting us, even if they don't know the exact right thing to say. It's good to know that they will always try to comfort us, no matter what happens.
You can offer to drive your wife to the hospital to see her sister or to drive her over to her parent's place. You can offer to do a few things around the house to lighten the load on her. You can make her a cup of tea and rub her shoulders. Sometimes it's the little practical things that really help.
Also you might like to tell her that you find it hard to say the right words in difficult situations. I think this would mean a lot. It shows that you are thinking of her and wanting to say something comforting.
It's hard when you feel pressure to be "strong". What does it really mean to be "strong" anyway? Sometimes, I think being strong could mean that you are strong enough to let people see your soft, emotional side. Strength can be in honestly opening up and saying that you are lost for words but you are there to help.
Strength may not be what some people believe it to be.... always being in control, always knowing exactly what to do and say. Strength for me, is admitting that you are human and that you don't have all the answers.