Your wife's wages can be garnished but not yours. Your joint account can also be garnished but if you have an account solely in your name it cannot. If you are concerned about this, then do not put a lot of money into a joint account. Pay bills from an account held only in your name. Nothing can be garnished unless and until there is a judgment against your wife. If your wife is sued, do not ignore the summons. She should go to court and try to enter into an installment order. An installment order would prevent garnishment, provided the terms of the order are complied with.
You are not liable on the student loan and therefore your wages cannot be garnished. Your wife if she had a judgment entered against her and if she was working could be garnished. In addition a joint bank account could be frozen and money levied by the creditor. If you file taxes together and the loan is in significant default such that the government takes it over then yes they can take your tax refund to apply it to the student loan that was defaulted. I would recommend you keep your finances separate as well and file taxes separately. If possible she needs to work out a payment arrangement with the student loan lender that is viable and affordable.
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