If her share of the settlement check is enough to make a down payment for a house, the amount must be significant.
The attorney cannot write a check to the client until the bank clears the check. For a large check, it may take weeks to clear the bank. Your sister should keep this in mind when she deposits her check. She may want to consider getting a wire transfer or some other quicker way of transferring money from the attorney.
If the attorney writes your sister a check and for some reason the funds from the settlement check are not available, the attorney likely will face discipline from agency that regulates attorney for letting his trust fund account be out of balance.