There are two ways that a marriage is absolutely void in Texas. One is the "Putative Marriage" where one party is already married to a different person. The other is that the parties are too closely related to each other. (TFC 6.201) The fancy word is "Consanguinity".
Remember that there are two types of invalid marriages: marriages that are void (discussed here) and those that are voidable. Your lawyer should not be sloppy and plead the wrong type of case. The term "annulment" is often casually tossed around by the client and the attorney, but that is only for VOIDABLE marriages. A void marriage never was. It does not get annulled. It gets a declaration that it is void. Such void marriages are properly filed as "A Suit to Declare Void the Marriage of ___ and ___". Also remember that unlike a normal divorce, there is no 60 day waiting period after filing suit. It is a small favor from the legislature that you can get these muddled messes over quick.
Remember that any type of marriage dissolution case has the same considerations if there are children involved; i.e. kids from these relationships are not "bastardized" by these proceedings. You have to do a full "Suit Affecting the Parent Child Relationship" and assign all rights, duties, access, visitation, etc. regarding the children.
Yes, this can get complicated quick. You should see a dedicated family lawyer if any of these issues are involved with your case. Many lawyers claim that they handle family law cases, but they only "dabble" when it is slow in their primary area of practice. If your attorney doesn't know the difference between "void" and "voidable" marriages, then you probably should find another lawyer. Proper pleading and initial fact gathering will save you a lot of hassle (and money) in these types of suits.