Cover letter to object the refusal of visitor visa to canada.

Hi is there anyone who could help me out with this issue.

I had applied for Canadian visitor visa two times in the past and they rejected my visa request. I want to write a cover letter to the Canadian high commission in UK or Canadian immigration, and I don't know how to write it. My visit is very important it's not for holiday or anything in fact I have a sister who is living there and she just got married two years ago and travelled to canada to live in there with her husband she moved there in 2017 and since then she had lots of family issues with her husband's family as she was living with them in the same house the family issues got worse and it became abusive and she has had a very difficult time as she is very young only 21years old and she is very alone in there and has no one to be with her and support her and what his husband did that when after marriage he applied spouse visa for my sister he didn't mention me as her sister in my sister's application. Due to her problems I had to apply visitor visa without telling the immigration the true purpose of my visit instead I told them that it is only a family visit I'm going with my husband and two children as my husband and two children are British they don't need any visa as they got ETA in seconds it's only me who need a visitor visa.
So my visit is very important to there to support my sister as she is pregnant and the immigration office of canada are not aware of this situation as I was afraid that it might affect my sister's status in Canada that she didn't mention it in her application as it wasn't her but her husband's mistake.

So now we come to the point why they refused my visa

Reason 1- I might not leave canada at the end of my stay as due to my immigration status in UK as I'm on spouse visa and in 1and half year I will get my indefinite.
Reason 2- that I might not live canada at the end of my stay due to my employment status. As I'm a house wife has two small children 1year old and 3years old and I host students in my house and my husband has a permanent job in Manchester airport and has no intention to stay in there.
Reason3- I might not leave canada at the end of my stay due to my travel history as I only travelled to UK after marriage from my country of birth.
Reason4- I might not leave canada at the end of my stay as purpose of my visit, because I didn't mention my blood related sister.

Can anyone guide me what to do to tell them the fact and how to write a cover letter for refusal of my visa as I don't agree with the reasons they rejected my visa for.

Thanks

.