If an owner of a vehicle loans a vehicle to another person they know is incompetent, reckless or unlicensed, they can be liable for their action.
The key is that the owner must know this before loaning the vehicle. Also, the owner must have given permission to use the vehicle. Express permission is when the owner says, "Take my car." Permission may be implied from the actions of the owner, such as a failure to object to the driver's use of the vehicle. Either type can be given.
Although driving records may be used to show the driver was incompetent, reckless or unlicensed, the record can't be used to show negligence at the time of the incident. In other words, just because someone has a record of a behavior, doesn't mean it was the actual cause of a current incident.
However, intoxication at the time of the loan may be sufficient to prove recklessness or incompetence.
Based on this, since the stepmother was the registered owner of the bike, she COULD be held liable for the accident if it can be proven that she knew the husband (father) was intoxicated, and especially if she loaned the vehicle knowing that his license was suspended.
Just because OP SAYS that stepmom "was fully aware" that Dad was under the influence of alcohol or prescription drugs doesn't prove that she actually knew. If that can be proven, and she allowed Dad to drive anyway, then it appears she can be assigned liability for the accident. And if she can be held liable for the accident, that does make a credible argument for wrongful death as a result.
Stepmom was not responsible for preventing your dad from driving drunk.
No, she wasn't. But she WAS responsible for allowing him to drive HER bike while drunk. That makes her at least partially liable for the accident and the resulting death.