First off - you should only have additional children if you and your wife both want them and can take care of them. If you really want to keep the family name - why not have one of your daughters use their middle name as your family name? And really how important is it to have a family name?
I have two girls and am perfectly happy with the two (even though I was used to boys having all brothers growing up). After the second was born, my husband immediately turned to me and said should we try for a third - meaning maybe a boy for the third. My answer - well you could end up like your brother if we keep trying - (he has 5 girls - no boys). That shut him up.