If her parental rights are dropped for the granddaughter, will they also be dropped for my grandson or will I have to go to court separately?

Each child is considered separately. I'm not clear from your question what type of court action the other family member is filing - it could be a guardianship (which does not terminate parental rights) or an adoption. Your best strategy would be to also file for legal custody if you have not done so. An informal arrangement means either biological parent can show up and take the child some day and you would be powerless to stop it without a court order. If you have a legal court order, look at your order. If it is custody (by grandparent intervention in a divorce or paternity suit), mother's and father's rights are not terminated. If it is guardianship, mother's and father's rights are not terminated. However, in both of these, mother would have to take you back to court and prove she is now fit to take back her son. If it is adoption, it is full and final termination of both parents' rights and they would only have the access to the child you allow, the same as your neighbor or friends do (if you allow visits, etc., is up to you). I hope this is helpful. You should consult directly with an experienced family law attorney to get more specific information on your situation. Best wishes.