Welcome to Just Answer
I am a Solicitor and will assist you.
What you need to know is that as a married spouse, your brothers wife is able to make a claim in relation to the matrimonial finances should they divorce.
The matrimonial finances includes all of the assets and liabilities for both of them and it does not matter that the asset that your brother shares with your mother is in his sole name and not his wife's name. This is still considered a matrimonial asset.
I appreciate that this what you want to hear but I do have to be honest with you.
If they have been together (including the length of their marriage and previously co-habitation) for over 10 years then the starting point for the division of the asset is going to be equality (In relation to your brothers share). If she is going to be the main carer for the children then she could argue a larger share of your brothers share - around 10% more.
It is important that it is established how your brother holds the property with your mother so as to establish what his share is.
Any other person can claim an interest in the property (in relation to the other relatives you have mentioned) under the Trust of Land Act if they have 1) paid a deposit, or 2) paid the mortgage, or 3) added value through renovation.
Going back to the situation with your brother - your situation is rather unusual given the other relatives living at the property and just because his wife is going to have a share of her husbands share does not mean that a court will agree that the house be sold so that this can be achieved now. That is going to depend on how much equity is in the house and if housing needs can be met elsewhere (especially for the children) by downsizing.
If your brother has any other assets then perhaps he could offset these by giving his wife a larger share to retain in the house.
Your brother needs to consider family mediation. This is actually a prerequisite before an application can be made to court by either spouse for an order in relation to the matrimonial finances.
Mediation is independent and will try and help them reach agreement without the need for court. If agreement can be reached at mediation then this can be prepared into a consent order and submitted to the court for approval when applying for decree absolute in divorce proceedings. If approved this becomes legally binding and would prevent any future claims being made.
Please kindly remember to rate positively by using the stars so that credit is received for helping you today. Your question remains open when you leave positive feedback and I will be able to answer your follow up questions for free for you