Detailed Answer

Short answer: Yes — heirs of a deceased sibling who inherited (or would inherit) an ownership interest in property can and generally must be included as parties in a partition action in Oklahoma. To add them you must identify who they are, show how title passed to them (or that they claim an interest), and either join them directly (by amendment and service) or proceed through the probate or “substitution” procedures the court requires when heirs or unknown parties exist.

How this works under Oklahoma practice (basic principles)

Start with two basic questions: (1) who owns the property now, and (2) what procedural status does any estate or probate have?

If the sibling died owning a share of the property and the estate has been probated (the will admitted or the administrator appointed), the distributed owners or the personal representative (executor/administrator) are the proper parties to a partition action.
If the sibling died intestate (no will) and no probate has been opened, the sibling’s interest passed by intestate succession to heirs under Oklahoma law. Those heirs are necessary parties if the court must divide or sell the land because they have a present or prospective legal interest.
If heirs are minors, incapacitated, or unknown, the court will protect their interests by requiring appointment of a guardian ad litem, requiring probate to determine heirs, or permitting service by publication or other substituted service consistent with Oklahoma practice.

Practical steps to add heirs to a partition case in Oklahoma

Below is a practical roadmap you can follow or take to an attorney:

Gather documents. Obtain the deceased sibling’s death certificate, any will, probate filings (if any), deeds showing the sibling’s ownership, and title search results. These documents show whether the sibling owned a share of the property and whether a probate estate already exists.
Determine ownership and heirs. If probate has been opened, the court records will show the personal representative and beneficiaries. If there is no probate, you’ll need to determine heirs at law (spouse, children, parents, siblings, etc.) under Oklahoma intestacy rules. If heirs are unclear, a probate petition to determine heirs may be necessary.
Amend the partition complaint or file a motion to join parties. If you already filed a partition complaint, file an amended complaint or a motion to join the heirs as parties. Attach supporting documents (death certificate, probate documents, affidavit of heirship or title search). Oklahoma courts permit amendment and joinder to bring necessary parties into an action.
Serve the heirs or their representatives. Proper service is required. If the estate has a personal representative, serve the PR. If the heirs are known individuals, serve each heir personally according to Oklahoma civil procedure. If heirs are unknown or cannot be found, ask the court to allow service by publication or other substituted service and, if needed, appointment of a guardian ad litem for missing or minor heirs.
If probate is needed, open a short probate or ancillary probate. Sometimes the simpler path is to open a probate or file a petition to determine heirs so that the court formally names heirs and/or appoints a PR who can be served for the partition action. This can streamline joinder.
Ask for protective procedures from the court. Request appointment of a guardian ad litem for minors or unknown heirs, and ask the court to set deadlines for appearance and claim. If an heir appears late or a party was not properly joined, the court can later address correction or reopening of the matter as needed.
Consider settlement or quiet-title alternatives. If adding numerous or scattered heirs is impractical, parties sometimes negotiate buyouts, obtain quiet-title orders after reasonable notice, or pursue partition by sale with the court’s directions about notice and distribution.

What to expect at hearing and possible court orders

The court will want to be sure everyone with a possible legal interest has notice and an opportunity to be heard. Typical court orders in this context include:

Ordering joinder of named heirs or the estate’s personal representative.
Allowing service by publication for unknown or unlocatable heirs after showing due diligence.
Appointing a guardian ad litem to represent minors or unknown parties.
Directing partition in kind if feasible, or authorizing sale and a distribution of sale proceeds according to ownership shares.

Consequences of failing to include heirs

If heirs who hold an interest are not named or not properly served, a judgment for partition could later be attacked as void or set aside as to their share. To avoid collateral attacks, courts prefer that all persons with a present or potential interest be properly joined or that the plaintiff obtain court authorization to proceed against unknown parties with adequate notice measures.

Where to find Oklahoma rules and forms

For statute text, local rules, and forms consult the official Oklahoma sites:

Oklahoma Legislature (statutes and bill text): https://www.oklegislature.gov/
Oklahoma Courts (court rules, forms, probate information): https://www.courts.ok.gov/

When to hire an attorney

Consider hiring an attorney if:

Heirs are numerous, unknown, or dispersed across states.
The deceased sibling’s title is contested or clouded.
Minors or incapacitated persons may inherit.
You need help drafting an amended complaint, completing service by publication, or coordinating probate and partition proceedings.

Disclaimer: I am not a lawyer. This is general information about Oklahoma civil and probate practice and is not legal advice. Laws change and local rules vary. For advice specific to your situation, consult a licensed Oklahoma attorney.

Helpful Hints

Start with a title search and death certificate — those two documents usually tell you whether a probate file already exists.
If a personal representative exists, serve that person instead of every individual heir; the PR can act for the estate.
If heirs are unknown, document your attempts to locate them (last known addresses, social media, relatives) before asking the court for publication service.
Ask the court to appoint a guardian ad litem for minors or unknown heirs early so the court record protects their interests.
Keep detailed proof of service and filings — partition judgments are easier to defend when the record shows everyone had proper notice.
If the heirs agree, consider a written settlement dividing the property or buying out shares to avoid contested litigation costs.
Use the Oklahoma Courts website for local forms and the Legislature website to review statutes that may apply to probate and joinder issues.