Short answer: Demand letter then E J E C T M E N T
Long answer:
There are 3 ways to recover possession of a real property:
Accion interdictal (AI), accion publiciana (AP), accion reivindicatoria (AR)
I believe the first one is the most relevant to your situation because AI is available in cases where repossession of property is sought WITHIN ONE YEAR from the time of unlawful possession. AI is covered not by our regular civil procedures, but by SUMMARY PROCEDURE under Rule 70 of the Rules of Court. Summary which means, it’s our favorite – A QUICKIE (no trial, decision will be based on position papers/affidavits submitted by the parties, within 1-2 months may decision)! And you file this with the inferior court = MTC or MCTC. Once unlawful possession has exceeded ONE YEAR, say bye bye to AI, and it will either be AP or AR, which shall be filed with the RTC and governed by the long and winding, ordinary Civil Procedure. You might wanna google the rules of court of the Philippines so you will be properly guided, or misguided, whichever is applicable, just in case.
So if your case is one that falls under AI, from when do you count/reckon the ONE YEAR? Depends on which kind of AI
Accion interdictal has 2 types, aptly initialed: F-E-U-D
1. Forcible entry (FE) – unlawful possession/occupation by another THROUGH FORCE, INTIMIDATION, THREAT, STRATEGY OR STEALTH.
2. Unlawful Detainer (UD) – withholding by a person from another for not more than a year of the possession of a land or building to which the latter is entitled after the expiration or termination of the former’s right to hold possession by virtue of a contract, express or implied.
Both involves unlawful possession/withholding of real property from the rightful possessor/owner for not more than one year.
Basic differences:
1. FE: possession is unlawful from the very beginning; UD: possession in the beginning is lawful but eventually becomes unlawful AFTER expiration of right to possess
Eg. You have a house in Tondo and then you bought and transferred to a house in Forbes Park.
If you abandon your house in Tondo and a family moves in without your knowledge, that is FE.
If you rented out your house in Tondo to your relatives for 12 months because you want to sell it next year and they kept possession beyond that period or if they are in default with their monthly payments, and despite your demand to vacate they retain possession, that is UD.
2. In FE, you need to prove that you had previous physical possession of the property; in UD, it is not necessary.
3. Since FE is unlawful from the beginning, DEMAND TO VACATE PREMISES IS NOT REQUIRED. But for UD, PREVIOUS DEMAND IS REQUIRED AND IS JURISDICTIONAL (ie., no previous demand, the court will have no jurisdiction over it).
AHA! So that’s where we count the 1-yr period!
FE = from actual entry, UD = from date of last demand/letter to vacate
I therefore conclude that your case falls under UD (occupation of the premises by your cousin was not by force or intimidation, but by your parent’s tolerance = implied contract/agreement) and thus, you should make your written demand to vacate (sample: I demand that you remove your fat ass and ugly face from my mother’s property, now na you b*tch!). If they don’t leave after the period you have set forth in your letter, that is when the ONE YEAR PERIOD OF UNLAWFUL POSSESSION BEGINS.
Additional reminders:
FEUD cases are REQUIRED to be first referred for BARANGAY CONCILIATION. If you skip that part and you directly filed a case in court, the case will be DISMISSIBLE (unless the other party failed to question the non-referral, which will be deemed a waiver) and the court will refer it back to the barangay. Bawal magshortcut! In other words – ipa-FACE-to-FACE mo muna bago kay public atorni!

Also make sure you file the complaint/petition with verification and certification against forum shopping, this is a requirement for all pleadings filed under the summary procedure.
After you filed the case and you get…
1. a FAVORABLE decision, you then move for IMMEDIATE EXECUTION (file a motion for the issuance of a writ of execution, your lawyer should know this). The sheriff will do the execution (ie, forcibly remove them from the premises) after you pay certain fees and padulas and if they insist to be pasaway at bumalik pagkatapos mapalayas, it will constitute INDIRECT CONTEMPT (disobeying a court order, criminal/penal in nature = kulong or multa ang aabutin nya!)-- unless they perfected an appeal previously and filed the corresponding supersedeas bond and deposits monthly rentals to the court. In which case, you will file a counterbond in order for the execution to continue. Like in poker, when your opponent places his bet, you raise it and double the amount at stake. PD, I don’t play poker (I just thought I will sound cool if I insert something about poker).
2. an UNFAVORABLE decision (ie, dismissed or olats ka), it could mean that the court thinks your case is not actually AI (no, not American Idol! Remember accion interdictal? Hayun o, nasa ^itaas, basahin mo ulit) because the defendants were able to show that they actually have real possessory rights over that property (like they have a super-duper-peksman-mamatay-man-secret-agreement with your parents previously that they are giving that property to them because they are far handsomer than you and your brothers) which would invalidate or at least put to question, the legality of the extrajudicial partition you executed amongst your siblings, then you will have to file another case for AR or AP, whichever is appropriate. Si lawyer mo na ang bahala jan. Actually ang advice ko, kumuha ka na ng lawyer, at dun ka maniwala. Nagpost lang ako para may mabasa kayong iba.

Capisce? Goodluck.

On topic post ends here.
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Whew, I’m on fire! To which, ff would say, with a loud snort: No vicces you’re not on fire. You’re just really HOT.

(oops, I think she just melted… or she bailed)
And speaking of ONE YEAR PERIOD…
Ngayon ay ika-365 na araw mula ng ako ay unang maging kasapi ng PMT. This place is so much better with me in it. Ff told me so, herself (and you know her, she is always right!)

*smugface*. Thatday that i signed up here, she started humming that line from Your Song – “how wonderful life is, now you’re in the world…”

#AngPagbabalik