First... let me say I think you have a great family arrangment! My sister is 12 years younger then I am... not by plan, but that is the way it worked out. We have just a tremendous relationship. There was never any jealousy or anything. We did so many things together when she was growing up. Now, that she is an adult and married as well.... we have a very deep close relationship.
Ok... 3 months ... you are at that point where most nursing moms and babies hit a 'stride'. Meaning your supply is matching her demand exactly. HOWEVER, many babies ALSO go through a growth spurt around 3/4 months. Ironic, huh?! So.... she may very well be trying to get more! Keep at it. Nurse frequently... that may mean every hour or two hours. I know... tough ... but it can be done. It won't last long. Promise. I went through a very very similar thing with my daughter. I thought I was not giving her enough... but it worked itself out.
Oatmeal is good too as someone else suggested and yes, make sure you are getting enough water and nutrition.
But you will rarely feel 'full' at this point.
If baby girl is gaining weight as she should, having an appropriate number of wet and soiled dipes.... then she is getting enough to eat.
There are ALOT of really great breastfeeding books out there. I find 'The Nursing Mother's Companion' by Kathleen Huggins, R.N., M.S very helpful. She goes through each 'baby stage' and what you can expect, trouble shooting etc... It's a great resource.
La Leche League is also a great resource.
Don't give up!!! You're giving your daughter a great start!