My mom had bad toxemia and sisters all had preclampsia. What are my risks at 18 with first baby?

>25% - really high. I would recommend starting baby Aspirin (80-81 mg orally daily) before conception (or by 16 weeks at the latest) to decrease your risk by 15% or more. Preeclampsia predisposes you to future hypertension and neurological deficits (albeit subtle). Maternal-fetal medicine consultation is paramount in properly managing your high-risk pregnancy.
High. Your risks can be reduced by early prenatal care by an OB md and if it develops, a maternal fetal medicine specialist, nutritional counselling, and fetal non-stress testing(nst) with a biophysical profile.