It is a culturally accepted practice which is on decline due to medical disadvantages .Even movies promote it in a big way. This is going to come up again and again by relatives too. If you hear it in front if you , then and there tell them not to do it as it makes it awkward for children . They will decide what to do when they grow up and most probably you and SIL will not have any role who they select as thier spouses. Anyway you are too far away for any of this to effect you too much , so handle it in a cool way. are you not from south , these things usually are commented when children are born and then it fizzles out naturally.