Sudden Sister

My mother and her mother are best friends, and her mother is on a business trip, so she stays at my house temporarily. This makes sense, right? But since we are not biological brothers and sisters, this matter is a bit delicate. I thought it was just an extra sister in the family who needed to be taken care of, until—— She naturally opened her mouth to catch the ham I fed her, asked me "Will you always stay with me" at Muji, and naturally hugged my neck while riding a bike. It was only later that I realized: This relationship seemed to have gone astray from the beginning. The question now is, how should I tell the two mothers that their dream "wedding" may really be on the agenda?