I'm only 19 years old and I have a finace who has a set of twins by another women. Her mother which is 43years old is slandering me on her myspace page. Calling my daughter names and calling me names as well. Everything she put on her page is untrue and the bad thing about it is she even put my street name of were I live at on there. It has hurt me in so many ways. And every wrote stuff about my husband which other people shouldn't even need to hear about. Can I sue for slander/libel even when its on her myspace blog? She called my daughter a snot noes little brat. And me other names as well. She shouldn't of did this to me cause I have not said nothing to her. Thanks

You are right she should not do this. But she can basically call your daughter snot nosed and things like that.
She can not put your street. You need to contact Myspace and make her remove it or tell them you may have to get a lawsuit on them as well if they don't.
If she isn't causing you personal problems like loss of money or physical or emotional problems then she can say pretty much say anything she wants. It is up to you to prove they are lies and they caused you or your family problems. If you sue for emotional problems often the Judge would want to see that you have gone to drs/therapy to prove your claim.

Giving away personal information, such as an address, in a public location without your consent is illegal.
Depending on your area, though, that may be your husband's call (the daughters to mom to grandmom connection) but still you need to get it removed.

Besides that, she is protected by the first amendment. Block her on MySpace, easy as that. Some people can be so immature.