Is this visa always valid until 2026, or once I first enter Canada, it would expire after 6 months?
As a multiple-entry visa, it will remain valid for entry until the end of the expiry date on the visa in 2026. Unless you become inadmissible and your visa is revoked, you are entitled to request entry to Canada on or before that date. As its name implies, it is valid for more than one entries so it does not cease to be valid after one entry.
You must arrive in Canada on or before the expiry date on your visa.
https://www.cic.gc.ca/english/helpcentre/answer.asp?qnum=417&top=16
You can enter with the visa on the last day of its validity and still be authorized to visit Canada for up to six months (at the officer's discretion).
Is the scenario valid for my visa: Can I visit my in-laws each year until 2026, as long as each stay is under 6 months?
Basically yes, but your admission into Canada and the authorized period of stay are ultimately the decision of the border agent processing your request.
It is important to remember that other than Canadian citizens, permanent residents and registered "Indians", no one has the right to enter Canada. The visa only entitles you to seek entry into Canada at a port of entry (e.g. airport). The agent will then decide if you will be permitted into Canada and how long you are authorized to stay.
Usually, a six-month period is permitted for most visitors. However, if the agent has doubts on your financial ability, if the stated intention of your visit is short, if you return to Canada soon after you left, or if your passport is expiring soon, the agent may fix a shorter period of authorized stay. If your authorized period of stay is less (or more) than six months, this information will be provided to you at the entry.
The agent is also empowered to deny you entry if they suspect that you will not leave Canada at the end of your authorized stay or will not abide other conditions (e.g. not to study or work in Canada). Frequent travel for long periods without a good reason may raise suspicions in this regard; but once a year to visit families are almost certainly fine.
