There are very few drugs that you cannot breast feed on. We normally take medications in milligram amounts and they are generally in serum and breast milk in nanogram amounts. 1 mg = 1,000,000,000 nanograms. Nanograms of drug in the milk are very small amounts.
Lisinopril is an angiotenisin converting enzyme inhibitor, ACE-inhibitor. This is a class of drugs that is very potent at promoting endovascular health and in particular reducing proteinuria. All patients with diabetes should be on and ACE-inhibitor. All patients with significant proteinuria should also be on an ACE-inhibitor.
At the doses that might be expected from breast milk, it is very unlikely that lisinopril would have an adverse effect on an infant. If the infant were to be critically ill with substantially reduced renal function - then there might be a concern. I routinely have patients on lisinopril postpartum. Specific data on lisinopril is limited. There is data on a related drug (enalapril) that suggests that transfer to the milk is quite low.
The benefits of ACE-inhibitors are large to you. The benefits of breast feeding are substantial for your baby. Theoretically, there is little cause for concern. There are no reports of injury associated with use. There is limited data on enalapril that suggest limited transfer.