My gynae has advised me not to carry any weights. Does anyone know how many kgs is dangerous? Is picking up my niece who weighs 10kg not safe?

As long as you are not overdoing it, it should be fine. Pick your niece up gently (not in a sudden/jerky motion). Like what Diana suggested, watch your posture, bend from the knees and keep your back straight. Lift her up by straightening your knees. If you experienced any pain or spotting, or fels uncomfortable doing that, then it’s better not to continue with carrying your niece. You can still show affection by holding her hands or cuddling her while seated.