Can my employer cut my hours because of my availability and give them to someone with less seniority than me?

Availability is usually a basis for an employer hiring, firing, or cutting hours.

The concept of seniority needs to be viewed based on whether you have some contractual rights to seniority (such as based on a union contract or employment contract) or whether you just happened to work at a job longer that someone else. 

In Washington, an employer can typically fire or cut hours without any reason, so long as it is not a prohibited reason by law; of course a union right or employment contract can give you more rights than basic Washington or Federal law in which case you might have a case to complain.

As always, my comments are only applicable to Washington State and are not a substitute for getting competent, local, and more comprehensive, legal help.

If you are not available when your employer needs or wants you to work, your employer could certainly cut your hours--your employer, not you, determines your hours, schedule, and shifts. You employer could also give your hours to someone with the availability it needs, even if that person has less seniority than you; indeed, if you are not available when the employer wants you, it could terminate you. There are some exeptions:

1) You have an employment contract setting your hours.

2) You are covered by a union or collective bargaining agreement which either sets your hours or protects you on the basis of seniority.

3) The stated reason (your availability) is not actually true, and you are over 40, and the reduction in your hours is actually discrimination against you because you are older than 40.

Apart from the above, however, the employer would seem to be able to do what you describe.