Well, how did it lose 4 qts over time?? That's alot before anybody noticed and it seems that your sister is in the know about checking oil level.

How many miles on the engine? If the car body/interior is perfect, it may be worth fixing.
I am sure you can find a crashed car with a decent engine. Is the car in San Diego??

__________________
1979 300D 220 K miles
1995 C280 109 K miles
1992 Cadillac Eldorado Touring Coupe 57K miles SOLD
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1979 240D 140Kmiles (bought for parents) *SOLD.
SAN FRANCISCO/(*San Diego)
1989 300SE 148 K miles *SOLD