Why does your ex want the children returned on a Sunday night? Is she making sure they are ready for the school week? Ensuring homework is done? Bath night?
Perhaps having been apart all weekend, she feel she needs to see them before she starts her working week. That doesn’t sound unreasonable. If Sunday night is important to her, could you not swap a different night in the week, making it equal?
Well done for rearranging work to allow more time with your boys, but how you ex provides wrap around care is up to her, as long as the children are safe and happy.