Continuing this conversation...

If I am in a same-sex marriage, me and my partner have 2 children. One of the children was conceived via artificial insemination from me and my partner's sister, and vice versa with the second child. How can I account for that in ancestry.ca?
When I try to add a child, the only options for selecting an alternate mother are my spouse or "unknown," but since my partner's sister is in my family tree, the person IS known. How can I make this link?