My mum is dying , and I think I recall her saying something about how if she died before my father my father couldn't sell the house because it would be of mutual property of him, my 2 brothers and I . However I am not sure if she said exactly that or If maybe they changed the will or something.. how can I know.. I mean is it rude if I ask my mother? Because I don't want her to think I'm praying for her death or something but I need to know.. because my father said he had in mind to sell the house when mum dies

Why do you feel you need to be the one to ask her? If anything I would think that your dad or your dad , you and your brother should all talk to her together.

I agree with NOhelp4u - start by asking your dad what he can tell you about your mother's will (if there is one) and what he thinks would happen to the property after she dies. For most families property belonging to husband and wife is titled with them as joint tenants with right of survivorship, which means when one dies the entire interest in the property goes to the other, independent of what the will may say. But it's possible that they may own as "joint tenants in common," where each owns half and each person's half interest passes on to heirs they choose per their will (or the laws of intestacy if there is no will). Your dad should certainly know how the property is titled.

If she did write a will, it is possible that she left it to all of you, or there were some contingencies about being allowed to live in it. I don't feel that it is appropriate to ask, but it depends on how you ask, I guess. It could be stressful for her if she hears that your father is talking about selling it, for example, when her will left it to all of you. When she does die, her will will be read, and it will take plenty of time (months) for the court to probate it before anyone can force anyone out or sell it (unless all who inherit are in agreement to sell it).
So try to stop worrying about where you will live. You will have time.

I agree with NOhelp4u - start by asking your dad what he can tell you about your mother's will (if there is one) and what he thinks would happen to the property after she dies. For most families property belonging to husband and wife is titled with them as joint tenants with right of survivorship, which means when one dies the entire interest in the property goes to the other, independent of what the will may say. But it's possible that they may own as "joint tenants in common," where each owns half and each person's half interest passes on to heirs they choose per their will (or the laws of intestacy if there is no will). Your dad should certainly know how the property is titled.

The use of the word "mum", meaning mother, is a tip-off that the OP may be in the UK (while "mom" indicates the venue is the US). If OP is British, JTWROS may not be as common as it is here (and here it is not as common as it could be either; because when in the process of home fiancing the owners are often not even consulted by the deed scriveners so as to give them the option).

Here is my thought on this. It would be inappropriate to ask her what is contained in the will. You can ask if one exists so in the event of her passing it can be found. But as far as the contents to me that is overstepping the bounds of decency.

As far as anything in the house when the time is right, I would help the dad go through her belongings and mention that this or that has sentimental value to you. Like, I remember the time that me and mum...

The use of the word "mum", meaning mother, is a tip-off that the OP may be in the UK (while "mom" indicates the venue is the US). If OP is British, JTWROS may not be as common as it is here (and here it is not as common as it could be either; because when in the process of home fiancing the owners are often not even consulted by the deed scriveners so as to give them the option).

I live in Malta, Europe

I also think I recall her telling me not to tell him, which means that if I do he might change the will or force my mother to..

I was going to reassure you about something you just said, but you found my first response inaccurate for no reason.
You are on your own - at least from me.