At 15 months she has never been on any antibiotics? There are 20 or so antibiotics in the penicillin family--and these are often the first line of defense for most bacterial infections. If she hasn't, good for you that she has been so healthy.
My pediatrician told me that while allergies are hereditary, they are not always allergen specific. So, your husband has a penicillin allergy, but even if she has an allergy it may not be to the same drug.
Me, for instance? Can't take regular penicillin without a mild reaction (that started later in life) but have taken some of the others in the family.
I feel for you. I weaned my daughter at 18 months and I was really conflicted. Since I worked full time and pumped for a year...I was really OVER the pumping. But I really loved the time we had together, especially when I would return from a busines trip. I think I was more upset about it than she was...now she just makes me nurse her dolls (and a stuffed cat...lol)
Either pump and dump--you can rent a hospital grade pump or talk to your pediatrician. There is no need to wean before you or your baby is ready. The health and mental benefits will continue for as long as you nurse. You can also time when you take your medication to your nursing schedule.
Amoxicillin peak excretion in milk is 4-5 hours after dosing and at pretty low concentrations. I got staph while breastfeeding and had to take some pretty heavy duty antibiotics, and while considered compatible, I wanted to avoid any other side effects than could occur in my daughter (like the diarrhea that she ALWAYS get with antibiotics).
When all of this has passed, since you are concerned about inherited allergies, you should have her skin-tested. It is good to KNOW for sure if she has a penicillin allergy as you never know when you'll have to make a trip to the ER for something else.
Ultimately, you have to make the decision, but I wish you luck. If you decide to wean, I know it is hard. Don't make it any harder on yourself by feeling guilty.