I?m not a lawyer but my DH is and so are friends of mine so I have a certain amount of insight into how things work at law firms.
Law firms notoriously don?t ?grow? good managers, due to the nature of the business and the kind of person that is successful. As you describe yourself as senior and the issue is promotion I imagine we are talking about partnership, yes?
You say your colleague is outstanding and deserves the promotion. Although you may have more PQE than her, it?s not always about that as I?m sure you know. It's not about 'turns' or a queue. Your bosses will have identified that they want to make your colleague a partner, so the actions they are taking will be about justifying that decision on paper. Because they are probably rubbish managers they are not handling it well, and are hiding from a proper conversation.
If you accept the decision as being taken the question is what do you want to do about it? Do you want to go for partnership next round? If so, that?s what you need to focus on.
You mention the idea of ?going to HR?. I?m all for people using HR for advice and if necessary complaining to HR about something that is wrong, but I really really think that in the context of a law firm, in a situation like this, going to HR would not only do nothing to improve the situation, but would probably worsen it. What do you imagine they could do to help you?
If you are going for partnership it?s not like a ?normal? promotion, it?s about owning and taking responsibility for the business. A couple of things strike me about your posts. You mention about wanting to be given more work and responsibilities, and concerns about work allocation.
If you want to be operating at partnership level you need to get out of that kind of thinking where you are 'given' work and opportunity, and get into the frame of mind where you go out and get it yourself. Quite aside from anything else, most firms who will considering bringing in external hires direct to partnership, or even as a senior associate, will expect you to bring a following with you. So both for your chances internally where you are, and if you want to move, you need to develop that following.
I would suggest a conversation with your boss where you say, ?cards on the table, I realise why this is happening, you?ve decided to make Jane Bloggs a partner, I think she really deserves it, she?s obviously really good. However I?m keen to be considered for next year and would like to discuss how to maximise my chances.'
Before you go to the meeting have a hard think about how you want to develop your own practice, perhaps identify clients you could target, either existing or new, areas of work that could be potentially lucrative, creative ways of marketing these, and come up with a business plan of some kind identifying how you can increase revenue for the firm over the next year, making yourself invaluable, and acting like a partner. Present this to your boss, and ask for support and input into what your plans are.
If you do what will be seen as childish whinging, you are not acting like a partner, and will not be treated as such. Presenting an action plan to improve things and showing some ownership and responsibility will be seen much more positively. It might not succeed in getting you partnership, obviously I don't know anything about the other circumstances that influence the decision.
It may all seem unfair, but I think there needs to be a certain amount of acceptance about how things work, it?s all about the money (it?s all about the dum dum da da dum dum!), and you need to identify how you can work within the ?system? to your own benefit.