Mazal tov!
It is preferable for your wife to cover herself when breastfeeding. This can be often be done by raising or unbuttoning her shirt from the bottom, throwing a shawl or baby blanket over her shoulder, or just sitting with her back to where you are. However, if she finds this difficult, she can continue to breastfeed as usual even if you may happen to see her. If you find this arousing, she should make additional efforts to cover herself, or you should leave the room.
You also should not pray or make blessings in her presence when her breasts are uncovered.