http://legal-dictionary.thefreedictionary.com/Will
will n. a written document [/b]which leaves the estate of the person who signed the will to named persons or entities (beneficiaries, legatees, divisees) including portions or percentages of the estate, specific gifts, creation of trusts for management and future distribution of all or a portion of the estate (a testamentary trust).[/b] A will usually names an executor (and possibly substitute executors) to manage the estate, states the authority and obligations of the executor in the management and distribution of the estate, sometimes gives funeral and/or burial instructions,
nominates guardians of minor children, and spells out other terms. To be valid the will must be signed by the person who made it (testator), be dated (but an incorrect date will not invalidate the will) and witnessed by two people (except in Vermont which requires three). In some states the witnesses must be disinterested, or in some states, a gift to a witness is void, but the will is valid. A will totally in the handwriting of the testator, signed and dated (a "holographic will") but without witnesses, is valid in many, but not all, states. If the will (also called a Last Will and Testament) is still in force at the time of the death of the testator (will writer), and there is a substantial estate and/or real estate, then the will must be probated (approved by the court, managed, and distributed by the executor under court supervision.) If there is no executor, named or the executor is dead or unable or unwilling to serve, an administrator ("with will annexed") will be appointed by the court. A written amendment or addition to a will is called a "codicil" and must be signed, dated and witnessed just as is a will, and must refer to the original will it amends. If there is no estate, including the situation in which the assets have all been placed in a trust, then the will need not be probated. (See: last will and testament, holographic will, testator, executor, probate, estate, guardian, codicil)
http://legal-dictionary.thefreedictionary.com/guardian
guardian n. a person who has been appointed by a judge to take care of a minor child (called a "ward") or incompetent adult personally and/or manage that person's affairs. To become a guardian either the party intending to be the guardian or another family member, a close friend or a local official responsible for the child's welfare will petition the court to appoint the guardian. In the case of a minor, the guardianship remains under court supervision until the child reaches 18.
Naming someone in a will as guardian of one's child in case of the death of the parent is merely a nomination. The judge does not have to honor that request, although he/she usually does. Sadly enough, often a parent must petition to become the guardian of his/her child's "estate" if the child inherits or receives a gift of substantial assets, including the situation in which a parent gives his/her own child an interest in real property or stocks. Therefore, that type of gift should be avoided, and a trust created instead. While the term "guardian" also may refer to someone who is appointed to care of and/or handle the affairs of a person who is incompetent or incapable of administering his/her affairs, this is more often called a "conservator" under a conservatorship. (See: conservator, ward)
The bolded part above is exactly what I told you. The judge does NOT have to honor the wishes of a will - that if they think another person is better suited to have custody over the child, then they can/will appoint a person different than the one you have chosen. There is no way to tell you that a judge will allow your sister-in-law to be the guardian/custodian simply because you want her to be.
