Without directly answering your question, your best bet is to go to Surrogates/Probate Court and ask to have the Executor taken off the Estate for failure to keep it whole while he/she marshals the assets and liquidates them and then distributes them. The Executor could be held personally liable if he/she doesn't do everything to keep the assets of the Estate intact and making money while the Estate gets processed. That means the Executor should be having that family pay rent while they live there in what is to be your house.
I suggest you hire an Estate Lawyer and go after both the Executor and the boyfriend and his family for back rent and for keeping you out of your mother's home.
The executor's duty is to carry out your mother's written instructions in her last will and testament, subject to a fiduciary duty to you and the other will beneficiaries. The executor may find oral instructions persuasive to the extent they do not directly contradict the will.
Even assuming they were not registered domestic partners, it is not necessarily accurate to say the boyfriend has no rights, even though he is not a will beneficiary nor heir. He is, at a minimum, a tenant. If there is no provision for him to continue living in the house under the will, the executor should give him notice and bring an unlawful detainer (eviction) action if he is unwilling to vacate voluntarily. He should be paying rent to the estate.
Assuming your mother owned the house free and clear when he moved in 6 months ago, it is very unlikely he would prevail if he argued he has a property interest in the house by virtue of WA's "committed intimate relationship" (CIR) line of cases. Six months is a very short period of time. In addition, the CIR cases only treat property acquired during the relationship as "community-like" property.
While the boyfriend should depart or be evicted, you and the other beneficiaries have no right to enter the house while it is in probate administration. The executor's duty is to administer it and (presumably) sell it from the estate for your benefit. Among his or her duties is a duty to make sure none of you damage the home or remove items from it. Why is it important to you to enter the house? If I were the executor, I would invite you to view it supervised by me. I would make sure the tangible personal items in the house were protected from removal by the boyfriend and the beneficiaries. That is the executor's duty to all of you.
Has the executor opened a probate and obtained Letters Testamentary? Is the executor represented by counsel? If you need information, you might ask the attorney. Within 3 months of appointment, the executor must file and provide to you an inventory and appraisement under RCW 11.44.015. If it is late, you can demand it and it has to be presented within ten days.
You can also request special notice of the proceedings in probate, so nothing will be done in court. without you knowing it. If the information is coming from the executor's attorney and the administration appears to be proceeding reasonably, it should not be necessary for you to get your own attorney as beneficiary. Only if it the information is not forthcoming or it appears things are not being done properly should you do that.