That's not how genetics work though, unless you are a clone of someone you don't have the exact same genetics, just strong tendencies, and even then if your mum got none after gaining 4kg all up you might still get them if you gain 40kg suring your PG. Do you not have a father? Don't you have half his genes? Did HIS mother not get them? Are your mum and aunt identical twins? Google it - it's strongly linked to genetics. Or just google on the structure of the skin. In order for any substance to reach the layer that tears and forms stretchmarks you would have to be injecting it either under your skin or into your bloodstream.

The only thing which can potentially make any difference is weight gain - i.e. if you have quite resilient skin which wouldn't have a tendency to stretchmarks and you gain 6-12kg during a pregnancy you're less likely to get them than if you gain 40kg. If the tendency is there all bets are off, i only gained 6kg and i got them, but then all my family get them.

Bx