Welcome to just answer. Let me handle it in two parts.
I agree, with an early age cancer in the family a screening is necessary and this is done by two ways;
a) Direct by the camera
b) 64 orXXXXX this is non invasive method.
2) Barium enema
The constipation alone is usually not the presentation or symptom of colon cancer, so that should not be worrying. The constipation severity is defined by the following criteria;
1) hard, lumpy stools;
3) a sense of incomplete evacuation;
4) sense of anorectal obstruction
5) stool frequency
If any two symptoms or more are present for at least 12 weeks, the diagnosis of chronic functional constipation is considered. Irritable bowel syndrome is considered as one of the prime diagnosis for chronic constipation.
High fiber intake (lot of vegetable and fruits), exercises, walking and plenty of fluids always are part of the management of the chronic constipation. People who do not respond to this and OTC laxative are advised to take the following medications by prescription;
1) Polyethylene glycol 3350 (Miralax)
3) Tegaserod (Zelnorm)
These agents are proven for their effectiveness at improving stool frequency and consistency in patients with chronic constipation.
Ideally you should consult a gastroenterologist who can perform the following investigations which are the mainstay of the evaluation and which will give true perspective of the cause;
1) Blood investigations do give the clue. A complete blood count, biochemical profile, serum calcium, blood glucose, and thyroid function tests are routinely recommended in the evaluation of patients with constipation.
2) Depending on these blood results further blood work is done; serum protein electrophoresis, urine porphyrins, serum parathyroid hormone, and serum cortisol levels, may be done to identify or rule out myeloma, porphyria, hyperparathyroidism, and Addison's disease. These causes are not common causes so clinical manifestation and physical examination clues are important.
3) Barium enema or barium meal with follow through study
4) Colonic transit time; it can provide a better understanding of the rate of stool movement through the colon.
5) Other investigations like Anorectal manometry, balloon expulsion test, Defecography etc. Use of colonoscopy / sigmoidoscopy / endoscopy is not advocated in chronic constipation, but in your case it can be used as a screening tool.
Please feel free for your follow up questions.