I'm traveling to Canada from Romania because my mom is sick so I don't know how long I need to stay. Can I enter Canada with a one way ticket as a visitor?
Officially you do not need a return ticket. However, also considering your situation, it will be very helpful to secure entry for yourself.
eTA is not a visa that guarantees you entry, but a preliminary clearance. Once you arrive in Canada, you will have to convince the border service officer, that you are a genuine visitor, fulfill the terms of the program and that you will leave Canada at the end of your visit.
In order to be convincing with that respect, a return ticket might be very helpful. See this official guidance (emphasis/redaction mine):
A valid visitor visa and travel document does not guarantee that you can enter Canada.
When you arrive in Canada a border services officer will greet you. The officer works for the Canada Border Services Agency (CBSA). The CBSA protects Canada's borders and points of entry.
The officer will ask to see your passport or travel documents.
You will not be allowed into Canada if you give false or incomplete information. You must convince the officer that you are eligible for entry into Canada. You will also have to convince the officer that you will leave Canada at the end of your approved stay.
The officer will stamp your passport or let you know how long you can stay in Canada. The period is usually six months. In some cases, the officer may limit or extend this period to cover the planned purpose of your visit. Ask questions if you are not sure about anything.
Basic travel requirements
You must meet some basic requirements to travel to Canada. You must:
- have a valid travel document, like a passport
- be in good health
- have no criminal or immigration-related convictions
- convince an immigration officer that you have ties—such as a job, home, financial assets or family—that will take you back to your home country
- convince an immigration officer that you will leave Canada at the end of your visit
- have enough money for your stay. [...]
As you can see the officer will make a judgement call on you, and there is no hard criterion like three months is OK, six months is not - instead the full picture is what counts. In your case, some doubt about your return could be in place, so having documents in support would be best practice. This could be a return ticket and proof of ties to Romania (family, job, education).
Finally, as remarked in a comment by @Aganju, typically return tickets are also cheaper than one-ways, so I don't see you loosing too much there.