If I was in your shoes, I would be honest and tell my SIL that the gifts requested are, all, way above the budget you allocate for the occasion (surely if this is not the first time you exchange Christmas gifts with each other she/they must know......?) and requests alternative (more appropriate) present wishes.
I would mention too that your maximum budget is £50, which for nieces/nephews is VERY generous in my mind (also for brothers/sisters!).
That way you don’t run the risk of a repeat next year........
We use (have done for years) Amazon Wish Lists in the Family for gifting, which is very convenient and practical.
Gift choice prices start from a few pounds upwards allowing for most budgets. . . . .
Good Luck and Happy Christmas.