it would be more difficult to get a visa for you if she was on benefits (thou some people have managed it), becuase the benefits she get would really only be enough for her and the baby, the embassy could say its not enough to support you and you would need recourse to public funds. there is no set figure she would need to show the embassy that she can support you all, but a figure of about £130 a week after paying rent and community charge would be a guide.
on a fiancee visa your not allowed to work until you have been granted FLR, as for you needing to look after the baby, she would probably be able to claim the childcare element of tax creds if you were not there, where the gov/taxpayer payes upto 75% of childcare costs.
as for her brother co-sponsoring you, until recenlty 3rd party financial support was not allowed, but there has been a ruling that a 3rd party could offer financial support, but i dont think its been implemented by the Home Office yet.so dont count on that.
you've not mentioned if you g/f has savings ? also where is she living ? what type of property, how many people live there ?
i would say its better if you waited til she has started work, has 6 pay slips and bank state ments , hopfully showing £2000+ in savings, with no statements over drawn and
that would increase your chances..
