You can absolutely drag money lent to a family member in court. The law doesn't care if it's a family member or not. The only thing that matters is that you are owed money, and that you can prove it.
If you lend someone money, and they don't pay it back, you have a few options. If the amount of money is small (like under $3,000), then you can sue them in small claims court. If the amount of money is larger than $3,000 but less than $25,000, then you would need to hire an attorney and file a lawsuit through the civil courts system. And if it's over $25,000? Then you might need to hire an attorney anyway!
The tricky part about dragging money lent to a family member in court is proving how much interest has accrued on that debt over time, especially if there isn't any written documentation of the loan agreement between the two parties involved (which would have been required if they'd taken out a formal loan). In these cases, it helps to have witnesses who saw both parties agree on terms of repayment at some point before or during the lending took place; otherwise, there may not be enough evidence for either party's case unless one side backs down first.