It's really hard to find someone who hasn't been shot down in flames when they suggest the MMR might cause problems. And to be honest, after the treatment received by Andrew Wakefield, any doctor / scientist may well think twice before doing so anyway.
When I was making the decision, I read all the mainstream stuff (IE NHS info, BUPA info etc etc) as well as all the anti vax stuff. I found it pretty much impossible to find any research that gives a definitive answer, and went with my gut instead, based on the known reported risks, and the personal stories of parents with once bright bubbly babies who have 'regressed' socially, mentally and physically after having the MMR. Rather than find something to prove it WASN'T safe, I failed to find any evidence that could guarantee me that it was.
I kept coming across the term 'reasonable risk'. Risk of temp spike, convulsions, ITP, intestinal disorders, neurological disorders, immuno disorders - you name it there's a risk of it. That on top of the many numerous reports of children being really, really poorly, just made me think there's no way I can knowingly put C at risk, just for the sake of having 1 jab instead of 3. It has never been my intention for C not to be vaccinated at all, hence the single vaccines - I just can't see how it can be of his benefit for his body to have to cope with all 3 at once.
I did get some useful info from these sites:
http://jabs.org.uk/
http://www.nccn.net/~wwithin/vaccine.htm
http://www.facebook.com/pages/Vaccination-Information-Network-VINE/69667273997
For what it's worth, I don't think your family is being fair to demand 'proof' from you. It is YOUR choice. It goes both ways anway,it's as hard to prove there isn't a risk as it is to prove that there is, so maybe they should proving to you that it's ok! Many studies are shown to be flawed, biased etc etc. If your instinct is telling you that you don't want the MMR, then no-one can force you. Not having the MMR doesn't necessarily mean not being vaccinated, and you'd do well to stress this to your family if that is their concern. I am lucky that OH is very supportive of my decision, as are my parents. The inlaws think I'm just a stupid hippy mom, but I really don't care