How much do we give?  

Weddings are very expensive and there really is no way around it. If you meant to give the gift to your sister-in-law as a wedding present, it should have been given at the reception. When it was given at the bridal shower, then it is viewed as a separate gift for that occasion. If your sister wants money and you don't have a lot to spend, then the best advice I can give you is purchase a savings bond for her. You can get them at any bank or financial institution and they are sold as EE series. They sell for half their face value so if you want to buy her a fifty dollar bond, it will only cost you twenty-five dollars. It will mature in a number of years, but she can put it away for the future which is a really good idea.

When we went to my nephews wedding last year we gave them a gift from their registry which was almost $100.00. They live out of state, so we didn't attend a bridal shower.

When we went to my nieces wedding, we gave them a check for $100.00 at the wedding. But we also went to her bridal shower and bought them a gift that was about $50.00.

Just give what you are comfortable with. I've never been to a wedding that has a money tree, so I'm not sure how that works.

A gift of 10,000.00 would be nice and she will love you for a wile. depending on how long the marige last

On top of a shower gift which is normal the normal according to a wedding book I had is how much you think your plate is per person, usally we give around 50.00 to 75.00, maybe a little more depending on if it is for family

So this is only my opinion but I think it is very tacky of your sister in law to ask for money as a gift. I usually do give money for the wedding and a gift for the bridal shower, but I don't feel it is appropriate for the bride to ask for money.
I would probably only give her like $25~ just because she asked! If nothing were said, I would give $50 or more depending on my finances at the time.

i think 40 dollars is suffice it's not too cheap and it's not too much especially since u gave her a gift already!

You gave her the wedding gift at her bridal shower. She should not expect money from you at all or anything else for that matter. I would have a hard time telling people that instead of buying me gifts, give me money. It is tacky to mention gifts at all. I would not put any money on the money tree.

If you already gave a gift then I think any amount should be good. Remember: You don't OWE them anything. You are giving them a gift out of the goodness of your heart and they should appreciate whatever you give them.

Whatever amount YOU think is sufficiant will do, after all: weddings, birthdays, and parties are NOT about money, it's about having friends and family join together and have fun. In a wedding's case, the creation of a bond between two loving couples.

A bridal shower gift is separate from an actual wedding gift. Money is often given for wedding gifts but... well, the bride and groom usually don't ask for it specifically. But that's not what you asked :-) so I think that $50 is what I'd give for most wedding gifts unless I was really close to the person in which case I may double it.