Is it permissible to marry one's maternal aunt's daughter if the aunt breastfed her sister's son once with an unfulfilling amount?

If you doubt the number of sucklings, then prohibition is not established by it. However, it is a precaution to avoid marrying the girl whose suckling is doubtful, as it clears one's conscience and keeps one further away from suspicion. It is advisable to look for someone else. She should be treated as a stranger, so it is not permissible to be in seclusion with her or to travel with her.