Is it legally allowed to marry my grandfather's sister's daughter's daughter? Will that affect our child's health?

It depends on the laws of your country, as marriage between close relatives is typically not allowed in many places. However, there are certain exceptions that could make it possible for you to marry your grandfather's sister's daughter's daughter (also known as a double first cousin). For example, in some countries like Finland and Turkey, a person can marry their double first cousin if they produce a certificate showing that both parties have received genetic counseling and tests proving they are not at risk of having children with significant birth defects or genetic disorders.

In terms of potential health risks for any future offspring, it is well documented that such unions can cause an increased risk for recessive conditions or gene-related abnormalities compared to nonconsanguineous (non-blood related) marriages. Therefore, if you were to decide on this type of union it would be important to take all necessary precautions beforehand in order to reduce any potential health concerns for your children.

It’s also worth noting that marrying a relative does not necessarily increase the odds of having abnormal babies; most couples in these situations have healthy children just like any other couple would. The best thing you can do is research your local laws thoroughly before making such an important decision and get clearance from two medical professionals regarding any potential genetic issues so you know what risks may be involved with this particular kind of marriage prior to entering into matrimony.