Unless you are a surviving spouse, you cannot roll someone else's retirement account (401(k), 403(b), IRA, etc.) into your own retirement account. You should have opened an Inherited IRA account and transferred her account to that. Moving your mother's retirement account into your 401(k) instead of an inherited IRA account means that you took a distribution of 100% of your mother's account, which is a taxable event. You will have to pay tax on the distribution. In addition, you need to talk to your 401(k) administrator about undoing the "rollover" to your 401(k) account.
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