I can't find reliable information.

She's not on the list in the official guide, or listed as an option on the UESP. I'd assume she can't be married.

On one wiki site, it says she can, but there are bugs with her daughter if you move from Whiterun (her daughter stays in Whiterun). While elsewhere, she can't be married at all.

i have never been able to marry her.

It's possible using PC console codes. I've tried it, but there are a couple of problems that you run into:

1) you need a key to her house which you can't get unless you pickpocket her, or add her to the potential followers faction and get one from her inventory
2) her bed is a double / queen size bed, but only one actor (including you) can use it at one time, and her daughter's bed is marked as "owned" (so there's no place for you to sleep if Carlotta's occuping it).

You could have her move in with you but her daughter would end up living in Carlotta's house alone. Unless you don't care about that (and since Skyrim doesn't have a Child Protective Service agency), that's a fesiable workaround.

If you don't know the console codes, I can provide them if you want?

I'm on the Xbox, but thanks anyways.

Well, that svcks. Chalk it up to another good NPC who can't be married.