As the court-appointed administrator, your wife's sister has the authority to hire the real estate professional. Unless the listing agreement she intends to sign is unfair to the Estate, this is probably not a fight worth having, particularly since everyone is agreement to sell the house. At the very least, I would suggest that the sales commission be 5 percent, and that if this person attempts to "double-end" the sale (representing both seller and buyer -- which I do not recommend -- even...
The short answer is yes, you need a probate litigator. The "computer" will witnessed by your half-brother and his wife is likely invalid, since beneficiaries may not serve as will witnesses. There are issues of undue influence involved in your fact situation; as well as issues of money tracing (the estranged wife).
Your sister in Taiwan has a right to the house. In order to be the 100% title owner, she will need to obtain a certified death certificate and have a translation of the death certificate prepared (and also certified). This then gets recorded in San Mateo County, showing she is the owner. I'm not sure what you mean by whether you, your younger sister, or your father have "any right to the house". If you all assisted with the down payment or payment of the mortgage, the answer is a definite...
It is possible to Petition the Court for the Administrator to provide a status report. Similarly, it is possible to Petition to the Court to require the Administrator to provide an accounting and/or to remove her. However, before rushing off to do so, you should consult with an attorney well versed in probate/trust litigation to review the case file.
So long as the reason you decide to not rent to them is not based on a discriminatory purpose, (race, gender, religion, etc.), you are not obligated to rent to anyone you don't want to, and you do not have to give them a reason.
You should put this property into a trust. The trustee would administer the property (or sell it if necessary) with the proceeds to go to your children. You are really asking an estate planning question, so should meet with an attorney that handles trusts and wills (estate plans) as soon as possible.
Bear in mind that the fee structure outlined by attorney Arvin are taxable as ordinary income. If you are a beneficiary and an executor, consider waiving the fees. The money goes back into the "pot" for overall distribution -- and as an inheritance, not taxable as ordinary income.
Generally, assets entitled as Joint Tenants with Right of Survivorship pass outside of the probate process and are not included in the valuation of an estate for attorneys' fees or personal representative's commission.
Most orders closing probates have a catchall phrase that permits the executor the power to find and distribute any after-acquired property. You may need to have a certified copy of letters testamentary re-issued. With the Letters, you should be able to apply for the funds, via the state controller's website. If you find it confusing, then use an attorney, who, at an hourly rate, will be able to wend his or her way through the thicket at a cheaper price than the company who contacted your sister.
Unfortunately, the "power" of a Power of Attorney ends when the grantor (your husband) dies. What you need is an emergency issues of Letters of Administration, or Letters Testamentary (if your husband had a will), that conveys the authority from the court, to act on his Estate's behalf. You may also need, depending on when the sale date is, the issuance of a Temporary Restraining Order to stop the sale, while you get your Letters of Administration.
Get to an attorney as quickly as possible!