My sister married to Canadian national

Dear All,
My sister married to a Canadian national in march 2019, my sister had a divorce from her first husband in 2015, and have 2 daughters. Now she married to a Canadian national who's applying for a visit visa for her only. Whereas he supposed to call his wife and the daughters on the same time. So my question is can a Canadian national apply for his wife a visit visa or he has to apply for the right category which is spouse visa.
Also please if you can help us how my sister's daughters age 7&5 can go to Canada with my sister.
Many thanks in advance.

He is not required to sponsor her.

With the invitation letter from her spouse, she can apply for a visitor visa for her and her children. It's unlikely it will get approved if they believe she will not be returning home.

Thankyou so much, she has applied for a visit visa on 30th may and haven't heard anything from the Canadian embassy yet. It has been 7 weeks.
Thank you