That is so sweet! I sang to my oldest (now 6) until he was about 3. It's not that I didn't want to sing to him, but I needed to take care of his little brother so I couldn't do it every night or for as long as he wanted me to. I do sing to him occasionally, just not all the time and not as often.
My youngest will not let me sing to him. If I try to sing to comfort him or relax him, he says, "Stop singing!" and will either cover my mouth or get mad if I don't stop. I do get away with it a little bit, otherwise I would have stopped trying altogether.
If it really concerns you, I would just talk to your oldest about it. I suspect you have some time before the situation would arise. Just let her know that you love this special time with her, but make sure she understands that she can fall asleep without them. Maybe she could sing a song or two with you one night and know that she could think of the songs if she's ever having trouble falling asleep.
I really do think it's wonderful. Cherish these moments!