Recently I have had consistently felt anger toward my boyfriends sister. It started last summer when she came up for lunch and just slated everything we cooked and my house. It annoyed me because she lives off the state and has not worked for nearly thirteen years. Although my house is not much it is all paid for and I am really proud of it. Its not just that though my boyfriend always goes out of his way for her to the point that sometimes it means we go without. Recently she made a comment about our finances and how she was worried yet she constantly asks to borrow money and never pays it back. I used to really get on with her but now I can't stand to be around her and everything she does angers me. I don't like feeling this way and I am sure she has no idea I feel this way. How can I get passed this please help.

If she hasn't worked for 13 years, she is either disabled or of low mental capacity- or she is scamming the state somehow. Reduce your anger by considering the source of the comments and realizing that her comments don't mean much.
The bigger issue is how your boyfriend's enabling of her and causing you to make sacrifices will affect your and his relationship and whether you will be able to endure that for very long.

She had a child so that is the main reason. I feel like I can't speak to him about it as he thinks I'm over reacting. I am just avoiding contact with her at the moment only last night he rushed out to give her money. I really feel like she is using us.

You need to separate 3 issues: how you and your boyfriend share expenses and income, your relationship with her, and his. His relationship with her is out of your realm, so concentrate on yours, hard as it may be to separate the two.
Money is a topic that can be tackled. If you and he pool your income, and own the house together, then put your foot down, and work out how much you each get to keep to do what you want with. Couples who aren't married really need to have very specific agreements about money, with a budget and categories and a ledger, to avoid the awful consequences if you break up. ESPECIALLY if you are the sole owner of the house!
You say she doesn't know how you feel - plan a few careful responses the next time she says something. Make it clear that you resent her criticisms, but do it short, sharp, and quiet.

Thank you so much for the advice

There is a need for her to know how you feel, and starting firm and polite would be how I would approach it, and go from there. It would sure help if you and your boyfriend would solve your issues and be a united front since she gets so much help from him. But this is something you can do yourself, or try to.

I am not very good at confronting people though. I have briefly discussed it with my partner who thinks I am being over sensitive. He also says that is just the way she is but I don't buy that. I will try my hardest to be more assertive. I just don't want it to come across as a battle for alpha female status.

You can certainly push back against unfair treatment though. How is standing up for yourself against a bully a competition? To be fair ignoring a fool is also a positive strategy, but you seem unable to do that too. It makes you angry.

Lol what ever you say or do risks being elevated,and escalated. Or you remove yourself from the conflict completely. Pick your option and follow through.

As long as you can afford to have this drain on your finances (or is it money he gives her that would help pay your joint expenses?), remain silent. I guess it could be worse if for instance you co-signed a loan for her.
Your boyfriend is a package deal which includes his sister and her child. Once you decide whether you can accept that, your anger issue will go away or be acted on.

To be honest the money is not a big issue its just the comments that anger me. We look after his niece often for full weekends but then get told what we should and shouldn't do. I know it must be hard for her and I suppose some of the comments might be out of jealousy. However my mum always said don't bite the hand that feeds you. Thank you all for your comments it's made me think about it from other perspectives.

There are three issues here; Boyfriend, his Sister's behaviour, by her borrowing money and her comments that affect your emotional well-being. In a relationship there are but two people I consider; myself and my partner. I later got this point across to my partner who would lend money without so much as discussing it with me or giving any forethought to our budget. Hence we go without… (He has since learnt how to communicate.)

When it comes to your adopted family; his Sister being the irritant of constantly borrowing money and giving slated comments. You later say; “To be honest the money is not a big issue it's just the comments that anger me.” Here I would beg to differ as previously “you used to get on with her…” This is a combination of factors that have built up over time which has not been addressed from the beginning. It starts like a small seed that gets watered and when it starts to grow without being pruned or weeded it grows into anger!

To rid yourself of this weed – anger; you'll have to toughen up a bit 44loo and approach Mr. Boyfriend and work out your financial priorities! It's great that you are debit free and proud of it; (as am I), but that doesn't mean you both haven't other goals to save up for? It also does not mean you are without compassion, to give a helping hand to family in times of hardship, but there comes a limit and a time when; one doesn't bite the hand that feeds you - her… :)

Plus you are by no means “being over sensitive” when his Sister is so rude with your hospitality, inconsiderate and financially interfering in you relationship!

Yeah you are so right. I feel a lot better and we are due to have a talk about this tonight amongst other things so fingers crossed he will see my point of view.