I did not use it verbally but in my own way: a request denied

[DavidH]

I applaud your not passing on the request, but I think that by lying about what you did, you are trading one unhealthy behavior for another.  I think a better course of action is to say Mother, I don't want to be and won't be in the middle of this, this is entirely between you and sister.  For any response from your mother than isn't :okay", I'd repeat, Mother, I'm not going to be in the middle of this.  You need to ask her yourself if it's that important to you.  This is an excellent time for bean dip.

It is perfectly possible to be polite about saying no to your mother, particularly in this case, but lying to her is not polite. 

[Craftymom]

OP, if your DM brings it up again, you can always give her a surprised look and say,
"You were serious? I thought you had to be kidding around."
"No, of course I'm not going to ask Sis about that, that would be ridiculous. Bean dip?"

[Luci]

That's one of those situations where I would blurt out, "You've got to be kidding!" to the other grandmother before I had time to control myself. Wow!

Maintain silence, "I didn't ask. It's your place if you really want to know", and if pressured I guess you have to honestly say, "That's not appropriate, " or even something about privacy and none of her goshdarned business. Well, maybe not that last, but really......!

[gollymolly2]

Whether the request itself is inappropriate or crazy is really not OP's decision to make. That's between OP's mom and sister.  Maybe for some reason OP's sister would be fine with her mom having this info.

The part of the request that was inappropriate was asking OP to be the go-between. If OP's mom wants this info, she should ask her other daughter herself. 

And that's why I agree with those who said you shouldn't lie to your mom about this. Just be direct and tell your mom to ask your sister herself. By implicitly lying and making her think you're going to find out the information, you're putting yourself even more in the middle than you need to be.

[TootsNYC]

I don't understand why you can't say "I don't feel comfortable asking her that.  If you want it why don't you ask her for it?"

Then when she says "Becuase she won't give it tome!" you can say "well then she obviously doesn't want you to have it and its not right to ask me to give it to youbehind her back expressly against her wishes".

If she argues, simply repeat "You need to take this up with sis".

This would probably be a good route--it might stop Mom from putting you in the middle.

Bcs gollymolly2 is right--it's not appropriate for your mother to do this.

Another tactic would be to say as often as possible, "Mom, I don't carry messages. Ask Sis this stuff yourself, and leave me out of it."

[whatsanenigma]

Maybe you could talk to your sister but not in the sense of trying to convince her to give mom what she wants.  Maybe you could just explain what mom has asked you to do and that you understand that this isn't your place and you don't want to get involved, but you want to find out how your sister wants to handle this.  She might want you to tell mom to ask her directly.  She might want to independently confront mom with no further involvement of you.  There are lots of different things she might prefer and and my opinion is that it would be best to explain this to her and ask what would work best for your sister.

[Pen^2]

That's one of those situations where I would blurt out, "You've got to be kidding!" to the other grandmother before I had time to control myself. Wow!

Maintain silence, "I didn't ask. It's your place if you really want to know", and if pressured I guess you have to honestly say, "That's not appropriate, " or even something about privacy and none of her goshdarned business. Well, maybe not that last, but really......!

I'd prefer to do this, personally. Just keep laughing about how funny and ridiculous the suggestion was. When they explain that they're serious, give them the benefit of the doubt: "Okay, it was funny the first time, but joke's over now. We both know you're not inappropriate enough to seriously ask something like that. Bean dip?" Insistence that they were serious is met with the same response. It gives them an out while making it clear that what they're asking is so out of the question that it won't even be entertained for a moment. Of course, you have to be prepared to look a little dim for constantly not understanding that they're serious.

But whatever you do, OP, I'd make it clear that you're not going to do what's being asked. Both so she won't keep bugging you about it, and so she won't ask for similar things in the future. Use whichever strategy fits best.

[Roses]

I agree with other posters, I don't think you should lie. I think you should get out of the middle and redirect with lots of bean dip.

Mom:  Tell your sister to get me the password to the school computer.
You:  Mom, if you need something regarding sisters kids, talk to sister.
Mom:  But she won't give it to me.
You:  I'm afraid it won't be possible for me to help you with this.
Mom:  Why not, I really want it?
You:  How's the weather at your house Mom?

[bopper]

If your mom follows up:  "Oh, I didn't think you were serious.  Asking for access to their grades is so inappropriate."

[MrTango]

Any time anyone ever asks me to be a go-between (outside of work, where that is sometimes part of my job description), I decline.  I don't play that game.

[BeagleMommy]

"Mom, anything you want to know about the grandkids has to go through sister.  I will not get involved."

Repeat, ad nauseum.

[Mikayla]

In a situation like this, silence is an indirect form of bean dip - just a very quiet one.  These work for one-offs, but if this is a pattern, then I like the suggestions PPs have given.  I'd have trouble hearing this and not pointing out that I stopped playing post office in 6th grade.