There was an Australian Story episode about a family that has Marfans sydrome and some of what they went through with diagnosis and operations etc. It is called 'All in the Family' and there is a transcript online. A big word of warning though....as others have said it can present in a wide range of severities and the family in that story have a more severe version (one family member was advised not to have IVF or carry a child due to the risk on her heart and having to stop medications etc.) so it gives some of the picture but much of what they talk about relates to their version of it which sounds very different to the version that may be in your family ie. they have had stuff that wouldn't have been able to be 'forgotten' or just not noticed as you have described with your family who have had children without issue.
I think the testing for it depends on what they already know about your in laws and their specific genetic 'mutation', though I am not an expert in this so I could be wrong. From what I understand if they already know where the mutation is it is a pretty simple test (blood test) but if they have to find the mutation first - which is possible the Drs know your family have the syndrome but don't know the specific mutation - it is a longer process...still a blood test but taking months to get a result rather than days or weeks. Either way a referral to a geneticist would be in order if you do want to get testing done. The geneticist I saw (in Adelaide) had a 6 months wait for non-urgent cases so it might be worth considering getting a referral JIC you do want to at some point.

Good luck...x