Divide the Original basis by 5 (Mom, Dad, Client, Sibling1, Sibling2). Gifting basis rules apply

Stepup 2/5 of basis to FMV at death and divide by three then add to each child's basis

Example: original basis $50,000 so $10,000 each

Step up FMV $100000 (Mom new Basis $20000 & Dad new Basis $20000) ($40,000)/3 = $13,333

Children's new basis $10,000 original plus $13,333 step up = $23,333