Oh bless you. It's bad enough to have someone die who you care deeply about. It's even harder when you can't say goodbye properly.
Would it help to tell us about your mam? What was she like as a person? hat did she enjoy doing?
Something that really helped me when I my mum died was sorting through pictures - I put together an album of pictures of her in all sorts of places. It reminded me of some things she enjoyed and that I had forgotten and felt a bit like doing a jigsaw - as if I were putting her back together again somehow, fitting all the bits together.
If you were to do something like that, it would give you a reason to contact your brothers, who might be able to send you some photos/memories, to add to your collection.
Some people find it helpful to have a special candle to light for her, perhaps just whilst you are waiting to scatter her ashes? You could have it by a photoframe of her, and light the candle on special days, or whenever you want to, as a way of showing that you are thinking about her. You could light it everyday for 10 minutes when the children are all in bed, and you have some quiet time.
Do think about bereavement counselling too. Cruse are wonderful and may well be operating online. They will listen. You can talk about your mum, and about how you feel and they will understand. They are well trained, but perhaps more importantly they have all lost someone close to them too, so know what you are going through.
Hope that helps a little. Thinking of you.