My FIL worked abroad and came home every fortnight. DH knew dad had a girlfriend, but she promised that she was't interested in marrying FIL. Surprise, surprise, we came back from our honeymoon to be told that FIL was divorcing MIL to marry GF. He expected us to break the news to MIL. We refused, and, when FIL did tell her, she was devastated. It estranged DH's from his father. Funnily enough, we really like step-MIL (younger than me!), and would really love to see her again. My advice is yes, meet GF - it's the problem of the 3 people in the marriage, not yours, and she may be a really lovely person who adores your son. My first BF was in a relationship when we met and he did love us both equally, but of course, she came first. At the time it was hell, but I always suspected that his partner and I would have been best mates had he not come between us being friends.