Welcome to the site,
You are absolutley correct that Pe can run in families. Its well documented that it can run along the maternal side but less well known is the increasing evidence which says it can also run down the paternal line too.
Lupus and other auto-immune conditions are very closely linked with PE. The current data says that if you have Lupus or APLS (anti-phospho-lipid syndrome) you are about 7 times more likely to get PE. So Im really glad to hear your sister has already been started on aspirin & clexane.
What we dont know is the 'accumulative' effect of the risks. So a strong family history would give you an increased risk of about 1 in 10 but we dont know (although we think it unlikely) that this would then be added to the 7 times increase because of APLS, giving an overall risk of 17 times more likely to get PE.
Neither does the increased risk give any clues in to the severity or gestation that one might actually get PE.
This is what is so hard about diagnosing PE - it can be so insdiscriminate.
I hope this helps, but do let us know if we can offer you further information.
With best wishes
Every Woman is entitled to understand what happened in her pregnancy when pre eclampsia strikes. I hope to be able to support that process.