Visit your in-laws only as often as you are comfortable, and allow their participation in your baby's life at the level that feels right to you. With your partner's support, your mother-in-law will come to accept her natural place as a grandmother. And in time, you will feel increasingly secure in your position as a mother.
Remember, too, that your mother-in-law is adjusting to sharing her child with his new family. If she is living vicariously through you it may have much to do with her loneliness in her own marriage. You cannot change this for her.
Though there may be some reaction from your mother-in-law as she adjusts to grandmotherhood, she will most certainly get over any feelings of rejection if you continue to include her in your life. Send her cards, invite her to visit you and the baby even if, for a period of time, she falls silent. Consider her frailty, but do not take negative "cues" from her. Act with the maturity of your new identity as mother and treat your mother-in-law with patience until she comes around. You and your whole family will benefit!