In physics forums, I created two accounts (actually for myself, for not being able to change my photograph), then I was suspended from the site. Now, as my sister is forcing me to create an account for her also, can create another for her? Why I asked is that, I should not get suspended even here.
Yes, you're allowed to create multiple accounts, but there are some restrictions on how you can use them. Let me quote the key part of that answer:
if the second account allows you to do something on the site that your normal account would be prevented from doing, it is abuse. Examples of this include (but are not limited to):
- Voting on your own posts
- Casting multiple votes on others' posts
- Supporting your own arguments ("+1: shog is right, don't know why the rest of you don't realize this")
- Using bounties to circumvent the rep cap
- Circumventing suspensions, quality bans, or the rate limits on posting questions / answers / comments / etc.
If the evidence available to moderators is suggestive of any of these activities, we'll take whatever corrective action is necessary. Usually that means merging and suspending the accounts, but of course it could vary on a case by case basis.
If you want to create multiple accounts and use them regularly, I'd suggest thinking carefully about why you want to do so, and whether you really need to. Although, as I said, it is allowed to create and regularly use multiple accounts, it's not very common to have a good reason for it.
Incidentally, Physics Forums is a completely different kind of site. In general, anything you may know about their policies won't necessarily apply here.