The Starwood AMEX might be of some help, since you can exchange Starwood points for Frequent Flyer miles on many programs at 1 - 1, with a bonus of 5,000 miles when you redeem 20,000 miles (i.e., redeem 20K Starpoints and get 25K FF miles). This works on most of the major international (LH, AF, BA) and US-based legacy airlines, but not on Continental or United (they require 2 Starpoints for 1 FF mile). Click here for the exchange rates.

And, Starwood used to allow people with the same mailing address to transfer Starpoints between their accounts. I think they still do, but can't say for sure.

So, if your parents and in-laws are eligible for a (US-based) Starwood AMEX card, and you have the same address as they do, they could earn Starpoints for paid stays at Starwood properties as well as for expenses such as purchasing airplane tickets that they charge to the AMEX card. If they transferred those Starpoints to you, you could convert the Starpoints into FF miles.

But I know of no way you can transfer Frequent Flyer miles they might earn from flying from the airline program into Starwood, so Starwood won't help convert the miles they earn from flying to visit you into FF miles in your account.