Do we have a wrongful termination case here?
No. When the employer is not a government agency, then the employer may legally fire you for any reason (or no reason at all) except for a few reasons prohibited by law. The prohibited reasons include firing you because:
• of your race, religion, sex, national origin, age, disability, or genetic test information under federal law (some states/localities add a few more categories like sexual orientation);
• you make certain kinds of reports about the employer to the government (known as whistle-blower protection laws);
• you participate in union organizing activities;
• you use a right or benefit the law guarantees you (e.g. using leave under FMLA);
• you filed a bankruptcy petition;
• your pay was garnished by a single creditor or by the IRS; and
• you took time off work to attend jury duty (in most states).
He wasn't eligible for FMLA in this circumstance, even if he had been there a year. Since he's not married to you at this time, the fact that you had to leave town to care for your sister doesn't really enter the picture here because neither your nor your sister are relatives of your fiancé. Thus, viewing it from your fiancé's circumstance, this falls into simply a child care issue, like any other single parent would have. Neither federal nor Colorado law offers an employee any protection from being fired because the employee's child care issues prevent him from doing the work the employer assigns him to do.
From your perspective, it seems "heartless." But the employer has work it needs done, and counts on its employees to be available to do that work. When they aren't available because of personal issues at home, that's not really the employer's problem. It costs money for employers to cover employees who can't do the work the employer needs, and as ClydesMom pointed out, firms can't accommodate every employee's personal needs in scheduling work. It's up to the employee to have in place plans to deal with personal issues that may arise. When you have kids, you have to put in place fall back arrangements for care when they get sick, or need care if another caregiver (e.g. spouse, fiancé, etc.) goes out of town, or whatever to ensure stuff like this doesn't happen. While it would be nice from the employee's perspective to have an employer who will accomodate every unexpected personal issue that the employee has, that's just not the world in which in we live.