Tough, especially considering you could get thrown in jail by being an accessory to a crime by knowing what her crime was and lying about it.
It would heavily depend on the crime. If it were something really bad and she got herself into deep trouble, I'd stay neutral and say I know nothing about any of it. That way, if her alibi is proven false, you don't go down with her. There's not much you can do to help her at that point.
If it was something minor, like a misdemeanor, I'd probably stick my neck out for my own mother. But again, it depends how deserving she is of the punishment and how serious of a crime she committed. No sense in going down with her if she dug herself too deep of a hole, whether she's your mother or not.