Why don't you go once your brother and niece have left? With regards to your niece putting your mum at risk - I think that's a decision for your mum to worry about - you can't be responsible for her choices. I imagine she's keen to see her. If it helps, my mum is very cautious but still saw my son (when we were allowed, in tier 2 currently) but kept a reasonable distance and was very careful. It is hard as we all have different views on what we feel is best.