Is it safe to take an antihistamine and a decongestant everyday to help combat chronic sinus infections?

In general, yes: Improving nasal function will decrease the incidence and severity of sinusitis. If you are allergic and congested then a combination of an antihistamine decongestant can be of help. So can intranasal steroids and other measures. If you have recurrent infections an allergy evaluation and a good ENT evaluation can be of great benefit.

Not for infections: If you have sinus infections these agents are not antibiotics. The may reduce secretions and dry upyour sinuses but will not kill bacteria. See your physician if infections are suspected.