Clients with family members with a special need, be it developmental, physical, cognitive or a mental illness, make up a significant number of clients, but the majority of advisors are not serving them, said Mary Anne Ehlert, a financial advisor and founder of special needs advocacy firm, Protected Tomorrows.

In fact, only a very small number of advisors understand the issues related to special needs well enough to do so, Ehlert said at her session, The Clients You Don't Know You Have at the FPA annual conference in San Antonio, Texas.

Even though people with special needs make up 20% of the population, according to the 2010 U.S. census, many advisors don't even ask clients about whether they have any family members with special needs.

Part of the problem is that advisors are afraid to bring it up, Ehlert said.

"You don't know what you don't know. You don't know that you should ask, and families don't volunteer it," she said.

So first and foremost, advisors must ask questions. Ehlert said advisors should find out if any family member, "up, down or around", has a disability. That means finding out if anyone, a parent, sibling, child or grandchild is dependent on the client and would need care if something were to happen to the client.

But she said, advisors don't have to ask directly if a family member is disabled, but can instead ask the client to tell them about that family member.

Once they've done that, it all comes out, she said.

"The issue you have to be aware of is that opens up Pandora's box because you gave them permission to talk about their biggest fear. So have Kleenex in the room and give it two hours," Ehlert said.

Besides not knowing about a client's special needs family member, the most important risk to avoid said Ehlert, is underestimating the amount needed to provide future care to the person with special needs.

According to Ehlert, the two most common mistakes advisors make are incorrect asset titling and incorrect estate plans that are inflexible or include the wrong documents.

If gifts or inheritances are given directly to people with disabilities, it could disqualify them from receiving government assistance.

But ultimately, it's very hard to know how to talk about and plan for special needs care if you've never been through it, Ehlert said.

Families can't do it alone and advisors often can't either because they simply don't know about all that's out there in terms of care.

She recommended that advisors work with specialists who can help them understand the issues around special needs care and quantify the costs so they can help families incorporate it into their financial plan.

"It's a combination of the family, and the advisor and some other resource, be it us or someone who can help them think through all those pieces that have to be plugged in," Ehlert said.