If an agreement was reached, than you and your dh have no obligation to pay for the activity. If biomom wants her to play so badly that she's willing to completely against the agreement, then let her pay, make it clear that your aren't willing to lose your Saturday time, and that's that.
My ex was adamant about dd playing soccer because that's the sport he played as a kid. My partner played softball and wanted her to try softball. Well, one season of soccer was enough for my daughter. She didn't like the game, and she didn't like that the girls got sat so the boys could win (and this was in the six and under division) and she hated her coach, who had two volumes to his voice: loud and louder. She loves softball, admittedly because more of her friends play softball, so she gets to see them.
When the next year rolled around, ex wanted her to play soccer again. Now, our child support is court ordered (which he fought tooth and nail) and it's $250 a month, based on the fact that when my dd was born, he was only making minimum wage. He now makes considerably more, but I haven't had the order modified because it just hasn't been worth the fighting that would ensue. He doesn't pay for anything else. So we pay for any activities. Well, realistically, we just can't afford two activities per year, so we gave her the choice and she didn't hesitate to pick softball. Ex, of course was certain that we were just not wanting her to play because he wanted her to.
Well, we told him we would take her to practices and games, and we'd even pay for her cleats and shin guards, but he had to pay her sign up fee. Not surprisingly, she didn't play soccer again.