The plus size bridesmaid dilemma

So I was probably one of the most excited people when my only sister got engaged.  I knew it was happening before she did.  So naturally she asked me to be her bridesmaid and I said yes.  I'm her baby sister why wouldn't I.  She was engaged for nearly 2 years and then in June this year she finally decided to set a date for her wedding.  It's going to be in may 2015.  I was so excited.  When Ali first decided that I was to be bridesmaid I was fine with it even when she said there would be five of us plus 3 flower girls and her matron of honour.  I never worried about it.  Then when she finally set the date the numbers had went down to 2.  Again I was totally excited about it. Then this week out of the blue my mum informs me that Ali is "panicking really badly" about "not being able to get a dress for me". Now if you have seen my previous blog posts you may have realised I'm a plus size girl and I'm not ashamed of that fact.  (So was my sister at one point). Anyway ... Since the first mention of bridesmaids I never once have had a concern about dresses even knowing that nat, her other bridesmaid Is a wee skinny thing.  So the fact I'm only hearing that she's panicking over the situation I kinda took offence.  At first I was like oh she's being melodramatic you can easily buy plus size dresses the same as other dresses.  But the way my mum keeps going on about it and that I might have to "get a different dress" or "its going to cost more money" etc is driving me mad.  Then I'm being told there will be 3 bridesmaids and that we will be going for our fittings in march 2014... And that I have to "lose weight" before then to get a dress I'm shocked and slightly panicked.  I'm now getting to the stage, thinking how am I gonna be able to lose 5 dress sizes in 5 months.  Plus is that even going to be enough.  I'm annoyed at the fact she's making me do this but more disappointed that she can't tell me it to my face but has to go through my mum.  But also I feel that she's ashamed of the way I look.  Yes I wanted to lose some weight before her wedding nearly 2 years away but I in no way thought I would be obligated to do so.  I know it may sound selfish and in some ways I feel a bit selfish but I honestly can't believe I'm in this situation at all.  I don't know what to do and now I just feel really down about the whole situation I really feel like saying I don't want to be part of her big day.  Am I just being a drama queen? Should I lose weight just to fit into her dress?  I'm just sad about the whole thing.  

I will post any developments 

Trying to stay strong 

Claire x