Dear OP - I have a lot of experience with this - my DS was a toddler like yours when my MIL moved in with us, and he is now about to leave for college. We've been doing this for the past 16 years, and I have learned a few things in those years (disclaimer - I realize that not everyone's experience will be the same, but apparently some of the posters have had similar experiences to ours):
1. Your relationship with your MIL will change when you are both living under the same roof. I had a great relationship with my MIL before she lived with us - but as soon as she moved in, in her eyes I was an incompetent wife, mother, cook etc - nothing I did was up to her standards.
2. The older woman will be the dominant woman in the household. As a friend of ours likes to say, "You can't have two queens in the same castle." My DH is very supportive of me, but she treats him like a 5-year old too.
3. If your DH's siblings are not involved now, don't expect them to be involved when MIL is older and may have greater medical/financial needs. My DH has 3 siblings, but when it comes to his mother's needs he is an only child. I am very thankful for DH's one sister who will take MIL for a couple of weeks a year so that we can go on vacation with just "our" family. That is the only thing that saves our sanity.
4. Your MIL is only 60 years old. You could easily be looking at a 30 year commitment here. You have to ask yourself honestly if that is something you can agree to.
5. You will be giving up a significant portion of your life (by this I mean not only you personally, but your life as a nuclear family) for your MIL's life. Every decision you make will be influenced by the fact that you have another person living in your house - and this will be true even if you buy a house like ours that has a separate apartment for MIL. My MIL is very "needy" - she relies on us not only for her financial support, but for all of her emotional needs as well. We cannot see friends without her being included, we cannot even go out to dinner or a movie by ourselves. She becomes incensed if any of our friends invite us anywhere without inviting her. Now that she is much older and has some mobility issues, and therefore cannot easily go everywhere that she used to go, she expects us to forego any event that she is unable to attend.
6. You will have no privacy, either as an individual, as a parent or a married couple. God bless the MIL who posted to say that she does not interfere in the parental decisions of her daughter and SIL - I wish my MIL could take a few lessons from her.
7. I strongly agree that we have an obligation to make sure that our parents are well taken care of as they age. This does not mean, however, that they must live in our homes to accomplish this. Both my DH and I agree that, if we knew 16 years ago what we know now, a senior residence close to where we live would have been a much better solution for all involved. That way, MIL would have had family close by to visit her, to take her places, and to see to her needs, but she would have had the opportunity to make friends and establish a life of her own instead of always expecting us to provide her social contacts.
You and your DH have to reach the decision that is best for you, but I wanted to give you some things to think about. This is not a decision to be taken lightly, as it will affect your family life for years to come.
Best wishes to you with your decision, and with your new baby!