Tell them it is in Allah's hands when to give parents the gift of a child, it is in Allah's hands to make that child a boy or a girl, it is in Allah's hands to give that child a healthy and happy life, we have no power over who we give birth to.
You should understand yourself that you are actually very fortunate to be in the position that you have a child on the way. For some people this doesn't happen, even if they plan for it.
At the end of the day, you cannot know that a boy would be good for you and your families future, rather than a girl. You cannot also know that neither you nor your brother will have any further children, as u said, this was unplanned, maybe you will have an unplanned son, or another daughter, you won't know this.
If it is in destiny for the male line to end here, it will, if Allah has planned for this male line to continue, it will. At the end of the day, any children that your daughters have will still have your family genes running thru them. You cannot also guarantee that if you had a son, that he would go on to have other children.
There are people without children in this world, who would be over the moon if they had either a boy or a girl. Take all your children as a blessing, and ask your parents to also recognise the blessings that you have.