Yes. All Celiac experts recommend that all first degree relatives, regardless of symptoms, are tested via bloodwork.
Something to keep in mind....just because they test negative now doesn't mean they are "free" of Celiac for life. If symptoms arise later, it is worth it to be tested again.
Here is some info you might find helpful, from a very well respected source.
http://www.celiacdiseasecenter.columbia.ed...nts/A02-FAQ.htm"Q: Why do my family members need to be screened for celiac disease?
When the diagnosis is established in one family member, their first- degree relatives (parents, siblings and children) should have blood tests for celiac disease. This is because at least 10% of family members will have the disease, even if they are asymptomatic. Screening is also recommended for second-degree relatives (grandparents, grandchildren, aunts, uncles and cousins) as they are also at an increased risk for celiac disease. Family members who already have an autoimmune disease are at a greatly increased risk to have celiac disease (25%).
The reasons to diagnose family members prior to the development of symptoms include the prevention autoimmune diseases and malignancy."