Alfie22,
IUI is basically artifial insemination. You are given either Clomid or probably in the case of your BIL/SIL, synthetic oestrogen/luteinising hormone by injections to encourage follicle growth.
Your SIL will be montitored using blood tests and scans every few days to determine when the follicle(s)are ready [i.e. the egg contained within is mature enough] then she will be given a trigger injection [of HCG which is the hormone that you produce when you are pregnant and it also triggers ovulation] and then asked to come in 24-48 hours later with BIl for insemination. Your BIL will provide a sample which is washed and then your SIL will be inseminated by catheter. The actual procedure is pretty much like a smear; where a speculum is used, the cervix is gently washed and a catheter containing the sperm is inserted. Then, you lie there or 10-20 mins and off you go.
I was told the chance of success was 20% [we were also unexplained] or in other words, the same chance as falling pregnant naturally if you got the timing perfectly.
Good luck to them.
Bubbs
