Hello Everyone.
First off I want to thank everyone for their advice and wishes.
We flew to see the FBIL and his FI on the weekend and chat about their wedding plans. We calmly put across our point of view on the option of them hosting their wedding the day before ours. We basically pointed out the timings and pressure on guests and the fact that we felt a bit strange them having an identical wedding.
They were really upset because they hadn't even thought about the guests and the fact that we may not like that they wanted the same wedding. FBIL's FI was so embarrassed and said that she just really liked our wedding and just wanted a really good fun wedding.
We had come prepared with some other isuggestions for them (red rock, shark reef, cloud 9 etc) but we also mentioned Hawaii (since everyone on FI's side is having a family holiday there on the way back). They loved the idea of Hawaii and I offered to help in anyway we can with the planning.
FMIL is also very relieved that everything has been resolved as I think she was in a really hard place trying to be happy for 2 sons.
So a good outcome in the end!