It's hard with the older sibling isn't it? I make sure the boys are occupied (with an activity, toys or TV) before putting DD down, then I put a CD on in her room and put the fan on so it makes a bit more white noise. This seems to help drown out her brother's noise!

I don't think the radio would interfere with your Angelcare monitor as it doesn't cause vibrations. You will probably here it through your unit though